PINTOSPATH should always point to the pintos src folder

fixed qemu path and added some kind of autodetection as multiple platform emulators are available on ubuntu 14.04

Recent clang version, detect attemps to dereference or call NULL, and
eliminate the code.

../../tests/userprog/bad-read.c:11:9: warning: indirection of
non-volatile null pointer will be deleted, not trap [-Wnull-dereference]
        *(int *) NULL);
        ^~~~~~~~~~~~~

This patchs adds the volatile modifier as necessary.
It allows to use clang to compile the tests.

On QEMU emulator version 1.7.0, the previous shutdown sequence (writing
`shutdown` to 0x8900), is not working properly.

This patch adds support for the ACPI shutdown code which works on qemu
1.7.0 and preserves the old sequence for compatibility.

Matthew Pirocchi <mrpirocchi@email.wm.edu>, via John Ousterhout
<ouster@cs.stanford.edu> reports that this fixes a problem on his system.

Fix gcc inline assembler operand constraints for syscall -- as pushl modifies %esp, variable operands must not reference the stack pointer.

Without this patch, gcc-4.6.1-9ubuntu3 miscompiles the read syscall to:
  000000b6 <read>:
    b6:   ff 74 24 0c             pushl  0xc(%esp)
    ba:   ff 74 24 08             pushl  0x8(%esp)  ; [!]
    be:   ff 74 24 04             pushl  0x4(%esp)  ; [!]
    ...

When compiled according to the instructions, bochs fails to load
the project 1 kernel.  This seems to be due to the latest gcc using an
instruction that bochs is not compiled to support.  Adding the
'--enable-cpu-level=6' configure option to Bochs, as this commit does,
seems to work.

bochs-2.2.6 failed to compile with the latest version of gcc.  This commit
fixes the problem

Reported by a student of Borja Sotomayor <borja@cs.uchicago.edu>.

This allows make to pass the proper command line arguments to the sub-make.

Reported by Francis Russell <fpr02@doc.ic.ac.uk>.

Reported by Francis Russell <fpr02@doc.ic.ac.uk>.

When the slave side of the pty is closed, the "read" system call returns
EIO.  In this situation, squish-pty failed to flush any output that it
already had in its buffer (up to 16 bytes) to stdout.  This could cause
the "Powering off..." message printed by Pintos just before exiting to
be missing from the output.

This usually didn't happen, because there are two other exit paths from
the relay() function that does most of squish-pty's work.  The most common
exit path was (evidently) the one which detected that the child process
had died, which did properly flush the buffer.  (The third path was wrong
too, and this patch fixes that one too.)

This patch fixes the problem by using a single exit path from relay() that
always reads any remaining input from the pty (if it is open) and flushes
it to stdout.

Reported by Borja Sotomayor <borja@cs.uchicago.edu>.

Reported by Borja Sotomayor <borja@cs.uchicago.edu>.

Some versions of ld accept /dev/null as a valid input file.  Others will
not.  The latter versions will give false negatives for --buil-id support,
which breaks linking of Pintos userspace programs.  So use a different
method, that I hope will be more reliable, to detect --build-id support.

Reported by Joseph Huang <jdhuang@stanford.edu>.

Here, $buf might have some data in it that has not yet been printed, but
we were discarding it (and trying to read more) without printing it.

(In Perl, "do" ensures that the inner block runs at least once; without
"do" the condition is evaluated first.)

Reported by Nathaniel Hardison <hardison@stanford.edu>.

Thanks to David Mazieres for reporting the problem and the fix.

Thanks to Godmar Back for reporting the problem and for the fix.

Rounding to nearest raises a host of issues that we don't want students
to worry about.

Suggested by Godmar Back.

The compiler can't optimize out the load of "ticks" here because
it can't look into intr_disable() and intr_set_level() and know that
they don't modify "ticks".

Thanks to Martin <mfleener@stanford.edu> for pointing this out.

We cannot expect that we know in detail ever interrupt that can occur
on real hardware, so we must cope with exceptional cases a little
better.

The Pintos loader writes status to the serial port as well as the
VGA console, but this doesn't work out so well on at least some
real hardware when nothing is connected to the serial port: each
character can take a second or so to display because the BIOS
waits for a serial timeout each time.  So, on the first serial
timeout, replace the "int $0x14" call by a pair of no-ops.

The rest of the changes are part of the struggle not to exceed
the maximum loader size.

Pintos used to contain a loader and I/O functions derived from MIT
code, but this has now been entirely eliminated, so there is no
longer any reason to retain the MIT copyright notice or license.

The contents of these functions was derived from code used in the
Massachusetts Institute of Technology's 6.828 advanced operating
systems course.

I deleted the contents of these functions and then rewrote them from
scratch without reference to the earlier code, working only from the
function comments (which I wrote earlier).

pinning is necessary
added FAQ for why stack growth may cause page faults above stack pointer
minor edits to other sections

http://gcc.gnu.org/onlinedocs/gcc/Return-Address.html )

added loadusersymbols to load user symbols from file
updated documentation to reflect behavior of qemu's and bochs's gdbstubs

This is in preparation for introducing new block devices, in
particular USB storage-based block devices so that Pintos can boot
from USB memory sticks on notebook and desktop PCs.

This block layer was inspired by one from Anthony Romano
<chz@vt.edu> but it has been extensively (perhaps entirely) rewritten.
Thus, bugs must certainly be blamed on the committer.

We now ignore pointers that are not null but still clearly wild.

We could check that there's really a valid PDE and PTE (I think Linux
does this) but it would take more code and be ugly.

Based on code from Anthony Romano <chz@vt.edu> but with extensive changes.

This makes it conform to the Pintos convention that a global symbol name
begin with its module's name.

This global variable was not named according to the convention that a
global variable name start with its module name.  Furthermore, its usage
was somewhat scattered across the source tree.

This commit introduces new functions shutdown() and shutdown_configure()
to improve the situation.