Revoke support for strxfrm() that write past the specified array length.

This formalizes a decision implicit in commit
4ea51cdfe8 and adds clean detection of
affected systems.  Vendor updates are available for each such known bug.
Back-patch to 9.5, where the aforementioned commit first appeared.

src/backend/main/main.c

@@ -149,6 +149,8 @@ main(int argc, char *argv[])
 	 */
 	unsetenv("LC_ALL");
 
+	check_strxfrm_bug();
+
 	/*
 	 * Catch standard options before doing much else, in particular before we
 	 * insist on not being root.