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/init/globals.c

@@ -40,6 +40,15 @@ struct Port *MyProcPort;
 long		MyCancelKey;
 int			MyPMChildSlot;
 
+/*
+ * MyLatch points to the latch that should be used for signal handling by the
+ * current process. It will either point to a process local latch if the
+ * current process does not have a PGPROC entry in that moment, or to
+ * PGPROC->procLatch if it has. Thus it can always be used in signal handlers,
+ * without checking for its existence.
+ */
+struct Latch *MyLatch;
+
 /*
  * DataDir is the absolute path to the top level of the PGDATA directory tree.
  * Except during early startup, this is also the server's working directory;