* 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.cc

@@ -258,6 +258,7 @@ proc_subproc (DWORD what, DWORD val)
 			    0, TRUE, DUPLICATE_SAME_ACCESS))
 	system_printf ("Couldn't duplicate my handle<%p> for pid %d, %E", hMainProc, vchild->pid);
       vchild->ppid = myself->pid;
+      vchild->uid = myself->uid;
       vchild->gid = myself->gid;
       vchild->pgid = myself->pgid;
       vchild->sid = myself->sid;