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().