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/utils/relmapper.h

@@ -29,7 +29,7 @@ typedef struct xl_relmap_update
 	Oid			dbid;			/* database ID, or 0 for shared map */
 	Oid			tsid;			/* database's tablespace, or pg_global */
 	int32		nbytes;			/* size of relmap data */
-	char		data[1];		/* VARIABLE LENGTH ARRAY */
+	char		data[FLEXIBLE_ARRAY_MEMBER];
 } xl_relmap_update;
 
 #define MinSizeOfRelmapUpdate offsetof(xl_relmap_update, data)