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/storage/bufpage.h

@@ -156,7 +156,7 @@ typedef struct PageHeaderData
 	LocationIndex pd_special;	/* offset to start of special space */
 	uint16		pd_pagesize_version;
 	TransactionId pd_prune_xid; /* oldest prunable XID, or zero if none */
-	ItemIdData	pd_linp[1];		/* beginning of line pointer array */
+	ItemIdData	pd_linp[FLEXIBLE_ARRAY_MEMBER]; /* line pointer array */
 } PageHeaderData;
 
 typedef PageHeaderData *PageHeader;