* sigproc.cc (no_signals_available): Use existence of signal thread handle to

* sigproc.cc (no_signals_available): Use existence of signal thread handle to
figure out if we can actually send signals rather than relying on my_sendsig.
(hwait_sig): Make static.
(sigproc_init): Don't set my_sendsig to anything special.  Use new global
static hwait_sig.
(wait_sig): Set hwait_sig to NULL when we are exiting.