* dcrt0.cc (sigthread::init): Reinstitute sigthread lock as a critical section.

(dll_crt0_1): Move sigthread lock initialization to earlier in startup.
* exceptions.cc (interrupt_on_return): Remove previous kludgy attempt to detect
an invalid frame.
(call_handler): Eliminate inner for loop.  Grab signal critical section lock
where appropriate.
* sigproc.cc (proc_subproc): Restore uid setting.
* sigproc.h (sigthread): Reinstitute sigthread lock as a critical section.
(sigframe): Grab the sigthread lock before clearing frame to avoid having the
signal thread use an invalid frame.

winsup/cygwin/sigproc.h

@@ -39,9 +39,7 @@ struct sigthread
 {
   DWORD id;
   DWORD frame;
-#if 0
-  muto *lock;			// FIXME: Use for multi-thread signalling someday
-#endif
+  CRITICAL_SECTION lock;
   void init (const char *s);
 };
 
@@ -69,7 +67,9 @@ public:
   {
     if (st)
       {
+	EnterCriticalSection (&st->lock);
 	st->frame = 0;
+	LeaveCriticalSection (&st->lock);
 	st = NULL;
       }
   }