* sigproc.cc (sigalloc): Don't set SA_RESTART here.

* sigproc.cc (sigalloc): Don't set SA_RESTART here.
* signal.cc (_SA_NORESTART): New flag.
(sigaction_worker): New function, derived from sigaction.  Don't set internal
flags unless called internally.
(sigaction): Use sigaction_worker.
(signal): Honor new _SA_NORESTART flag.
(siginterrupt): Set _SA_NORESTART flag appropriately.  Use sigaction_worker to
set flags.
* include/cygwin/signal.h: Define _SA_INTERNAL_MASK here.