Clean up maintenance of config table sentinels

Xundump had special hackery to maintain configuration tables'
sentinels: xubody() and getobj() added a sentinel element when
initializing or growing a table, which xubody() stripped off again
before returning.  The latter was an unclean hack.

Replace this by building knowledge of sentinels into struct empfile:
new flag EFF_SENTINEL, set for the appropriate members of empfile[],
obeyed by ef_extend() and ef_truncate().

src/lib/global/file.c

@@ -86,7 +86,7 @@
     SZ((array)), 0, SZ((array)) - 1, SZ((array)) - 1, -1, NULL, NULL
 
 /* Common configuration table flags */
-#define EFF_CFG (EFF_PRIVATE | EFF_MEM | EFF_STATIC)
+#define EFF_CFG (EFF_PRIVATE | EFF_MEM | EFF_STATIC | EFF_SENTINEL)
 
 struct empfile empfile[] = {
     /*