Verify table uid sanity more tightly

Verify table uid sanity more tightly

verify_row() refrains from rejecting zero uids, because some tables
may contain blank entries, with zero uid.

Change it to check only header sanity for entries that are not in use.
This filters out all legitimately blank entries.  Tighten up the uid
check.

For computing "in use", factor empobj_in_use() out of xdvisible().
Note that xdvisible()'s case EF_COUNTRY doesn't bother to check
nat_stat, because that's implied by what it does check.  It's not
implied in empobj_in_use(), so add it there.