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/commands/tablespace.h

@@ -25,7 +25,7 @@
 typedef struct xl_tblspc_create_rec
 {
 	Oid			ts_id;
-	char		ts_path[1];		/* VARIABLE LENGTH STRING */
+	char		ts_path[FLEXIBLE_ARRAY_MEMBER]; /* null-terminated string */
 } xl_tblspc_create_rec;
 
 typedef struct xl_tblspc_drop_rec