sync snd_nxt[2]: sync seq# via snd_nxt instead of write_seq

(1) call ptcp_rxtx_sync_shared_seq in tcp_write_xmit instead of tcp_sendmsg
(2) modified ptcp_enter_loss version which iterates on write queue only
until snd_head (since packets on the queue beyond that have been
generated in tcp_sendmsg, but not yet transmitted, so should not be
considered lost.)