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.h

@@ -30,6 +30,7 @@ void threadname_init ();
 HANDLE __stdcall makethread (LPTHREAD_START_ROUTINE, LPVOID, DWORD, const char *);
 const char * __stdcall threadname (DWORD, int lockit = TRUE);
 void __stdcall regthread (const char *, DWORD);
+int __stdcall iscygthread ();
 
 #ifndef DEBUGGING
 # define ForceCloseHandle CloseHandle