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/sigproc.h

@@ -18,8 +18,7 @@ enum procstuff
   PROC_CHILDSTOPPED	= 2,	// a child stopped
   PROC_CHILDTERMINATED	= 3,	// a child died
   PROC_CLEARWAIT	= 4,	// clear all waits - signal arrived
-  PROC_WAIT		= 5,	// setup for wait() for subproc
-  PROC_SIGCHLD		= 6	// saw a non-trapped SIGCHLD
+  PROC_WAIT		= 5	// setup for wait() for subproc
 };
 
 typedef struct struct_waitq