main.py: forward SIGTSTP/SIGCONT so "C-z" will actually suspend everything.

Catch and forward SIGTSTP (as SIGSTOP) and SIGCONT to the subprocess
as we already do for SIGTERM and SIGINT so that the subprocess will
also suspend/resume.

This still leaves bup with potentially unexpected behavior since the
(detached) subprocess will never see a SIGSTOP delivered to the parent
(because SIGSTOP can't be intercepted and forwarded).  This is due to
the os.setsid() call that was originally introduced to support current
newliner arrangement (cf. b7a524ccb662c9ed3ebd786da0f45f459929ef45).

Signed-off-by: Rob Browning <rlb@defaultvalue.org>