Check for process group change in tty_wait_background().

Check for process group change in tty_wait_background().

The calling process's process group can change between PROC_UNLOCK(p)
and PGRP_LOCK(pg) in tty_wait_background(), e.g. by a setpgid() call
from another process. If that happens, the signal is not sent to the
calling process, even if the prior checks determine that one should be
sent.  Re-check that the process group hasn't changed after acquiring
the pgrp lock, and if it has, redo the checks.

PR:	250701
Submitted by:	Jakub Piecuch <j.piecuch96@gmail.com>
MFC after:	2 weeks