Use FLEXIBLE_ARRAY_MEMBER in a bunch more places.

Replace some bogus "x[1]" declarations with "x[FLEXIBLE_ARRAY_MEMBER]".
Aside from being more self-documenting, this should help prevent bogus
warnings from static code analyzers and perhaps compiler misoptimizations.

This patch is just a down payment on eliminating the whole problem, but
it gets rid of a lot of easy-to-fix cases.

Note that the main problem with doing this is that one must no longer rely
on computing sizeof(the containing struct), since the result would be
compiler-dependent.  Instead use offsetof(struct, lastfield).  Autoconf
also warns against spelling that offsetof(struct, lastfield[0]).

Michael Paquier, review and additional fixes by me.

src/include/replication/walsender_private.h

@@ -54,7 +54,7 @@ typedef struct WalSnd
 	 * Pointer to the walsender's latch. Used by backends to wake up this
 	 * walsender when it has work to do. NULL if the walsender isn't active.
 	 */
-	Latch		*latch;
+	Latch	   *latch;
 
 	/*
 	 * The priority order of the standby managed by this WALSender, as listed
@@ -88,7 +88,7 @@ typedef struct
 	 */
 	bool		sync_standbys_defined;
 
-	WalSnd		walsnds[1];		/* VARIABLE LENGTH ARRAY */
+	WalSnd		walsnds[FLEXIBLE_ARRAY_MEMBER];
 } WalSndCtlData;
 
 extern WalSndCtlData *WalSndCtl;