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/varbit.h

@@ -26,7 +26,8 @@ typedef struct
 {
 	int32		vl_len_;		/* varlena header (do not touch directly!) */
 	int32		bit_len;		/* number of valid bits */
-	bits8		bit_dat[1];		/* bit string, most sig. byte first */
+	bits8		bit_dat[FLEXIBLE_ARRAY_MEMBER]; /* bit string, most sig. byte
+												 * first */
 } VarBit;
 
 /*