Add a default local latch for use in signal handlers.

To do so, move InitializeLatchSupport() into the new common process
initialization functions, and add a new global variable MyLatch.

MyLatch is usable as soon InitPostmasterChild() has been called
(i.e. very early during startup). Initially it points to a process
local latch that exists in all processes. InitProcess/InitAuxiliaryProcess
then replaces that local latch with PGPROC->procLatch. During shutdown
the reverse happens.

This is primarily advantageous for two reasons: For one it simplifies
dealing with the shared process latch, especially in signal handlers,
because instead of having to check for MyProc, MyLatch can be used
unconditionally. For another, a later patch that makes FEs/BE
communication use latches, now can rely on the existence of a latch,
even before having gone through InitProcess.

Discussion: 20140927191243.GD5423@alap3.anarazel.de

src/backend/utils/misc/timeout.c

@@ -284,11 +284,9 @@ handle_sig_alarm(SIGNAL_ARGS)
 
 	/*
 	 * SIGALRM is always cause for waking anything waiting on the process
-	 * latch.  Cope with MyProc not being there, as the startup process also
-	 * uses this signal handler.
+	 * latch.
 	 */
-	if (MyProc)
-		SetLatch(&MyProc->procLatch);
+	SetLatch(MyLatch);
 
 	/*
 	 * Fire any pending timeouts, but only if we're enabled to do so.