* cygthread.cc (cygthread::release): Reset ev here if it exists.

(cygthread::terminate_thread): Eliminat racy code which reset ev and
thread_sync.  Remove a few nonsensical inuse checks.  Exit at the bottom.
(cygthread::detach): Rewrite to again try to ensure that we don't say we're
signalled when we are not signalled.
* fhandler.cc (fhandler_base::raw_read): Revert to signalling read success
quickly.
* pipe.cc (fhandler_pipe::close): Use base method to close handle.
* sigproc.h (WAIT_SIG_PRIORITY): Just trundle along at normal priority to allow
the pipe thread to do its thing if possible.
* pinfo.h (pinfo::zap_cwd): Declare new function.
(pinfo::zap_cwd): Move 'cd out of the way code' here.
(pinfo::exit): Use it here.
* spawn.cc (spawn_guts): And here.

winsup/cygwin/sigproc.h

@@ -88,7 +88,7 @@ extern char myself_nowait_dummy[];
 
 extern struct sigaction *global_sigs;
 
-#define WAIT_SIG_PRIORITY THREAD_PRIORITY_TIME_CRITICAL
+#define WAIT_SIG_PRIORITY THREAD_PRIORITY_NORMAL
 
 #define myself_nowait ((_pinfo *)myself_nowait_dummy)
 #endif /*_SIGPROC_H*/