Fix final round of gcc warnings relating to unused parameters.

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.