* exceptions.cc (handle_sigsuspend): Just sleep forever if called from non-main

* exceptions.cc (handle_sigsuspend): Just sleep forever if called from non-main
thread.
(sigpacket:process): Simplify logic which determines when and how a signal is
masked.  Don't trigger sigwait if there is a signal handler.
* sigproc.cc (wait_sig): Update comment.  Try to process a signal which is in
the queue if it isn't queued for the target thread (this is still not right).