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/replication/syncrep.c

@@ -162,7 +162,7 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
 		int			syncRepState;
 
 		/* Must reset the latch before testing state. */
-		ResetLatch(&MyProc->procLatch);
+		ResetLatch(MyLatch);
 
 		/*
 		 * Try checking the state without the lock first.  There's no
@@ -235,7 +235,7 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
 		 * Wait on latch.  Any condition that should wake us up will set the
 		 * latch, so no need for timeout.
 		 */
-		WaitLatch(&MyProc->procLatch, WL_LATCH_SET | WL_POSTMASTER_DEATH, -1);
+		WaitLatch(MyLatch, WL_LATCH_SET | WL_POSTMASTER_DEATH, -1);
 	}
 
 	/*