Fix final round of gcc warnings relating to unused parameters.

* debug.cc (iscygthread): New function.
* debug.h: Declare it.
* exceptions.cc (set_process_mask): Flush pending signals.
(handle_sigsuspend): No need to flush pending signals.
(call_handler): Refine previous tests of muto ownership.  Only clear wait()'s
when we have definitely responded to a signal.
* fhandler_console.cc (fhandler_console::read): Don't set EINTR if executing in
a "cygwin" thread.
* sigproc.cc (proc_subproc): Use second argument to control whether CLEARWAIT
actually sets "signalled" flag.
* sync.h (muto): Add 'unstable' method.

winsup/cygwin/debug.cc

@@ -47,6 +47,17 @@ regthread (const char *name, DWORD tid)
   unlock_threadname ();
 }
 
+int __stdcall
+iscygthread()
+{
+  DWORD tid = GetCurrentThreadId ();
+  if (tid != maintid)
+    for (DWORD i = 0; i < NTHREADS && threads[i].name != NULL; i++)
+      if (threads[i].id == tid)
+	return 1;
+  return 0;
+}
+
 struct thread_start
   {
     LONG notavail;