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/fhandler_console.cc

@@ -151,7 +151,8 @@ fhandler_console::read (void *pv, size_t buflen)
 	case WAIT_OBJECT_0:
 	  break;
 	case WAIT_OBJECT_0 + 1:
-	  set_sig_errno (EINTR);
+	  if (!iscygthread ())
+	    set_sig_errno (EINTR);
 	  return -1;
 	default:
 	  __seterrno ();