* DevNotes: Add entry cgf-000019.

* dcrt0.cc (do_exit): Just set exit_state to ES_EVENTS_TERMINATE and nuke call
to events_terminate which just set a superfluous flag.
* sigproc.cc (signal_exit_code): New variable.
(setup_signal_exit): Define new function.
(_cygtls::signal_exit): Remove accommodations for closing the signal pipe
handle.
(exit_thread): Just sleep if we're exiting.
(wait_sig): If signal_exit_code is set, just handle bookkeeping signals and
exit ReadFile loop if there is nothing more to process.  Call signal_exit at
end if signal_exit_code is non-zero.
* sigproc.h (setup_signal_exit): Declare new function.
* exceptions.cc (sigpacket::process): Use setup_signal_exit to control exiting
due to a signal.
(exception::handle): Ditto.  Query exit_state rather than defunct exit_already
to determine if we are exiting.
* globals.cc (ES_SIGNAL_EXIT): New enum.
* sync.h (lock_process::release): New function for explicitly unlocking muto.
(lock_process::~lock_process): Use release method.

winsup/cygwin/sigproc.h

@@ -89,6 +89,7 @@ void __stdcall sigalloc ();
 int kill_pgrp (pid_t, siginfo_t&);
 int killsys (pid_t, int);
 void exit_thread (DWORD) __attribute__ ((regparm (1), noreturn));
+void setup_signal_exit (int) __attribute__ ((regparm (1)));
 
 extern "C" void sigdelayed ();