Avoid using NAMEDATALEN in pg_upgrade

Because the client encoding might not match the server encoding,
pg_upgrade can't allocate NAMEDATALEN bytes for storage of database,
relation, and namespace identifiers.  Instead pg_strdup() the memory and
free it.

Also add C comment in initdb.c about safe NAMEDATALEN usage.

src/bin/initdb/initdb.c

@@ -1836,7 +1836,7 @@ setup_collation(void)
 #if defined(HAVE_LOCALE_T) && !defined(WIN32)
 	int			i;
 	FILE	   *locale_a_handle;
-	char		localebuf[NAMEDATALEN];
+	char		localebuf[NAMEDATALEN]; /* we assume ASCII so this is fine */
 	int			count = 0;
 
 	PG_CMD_DECL;