Inplement the --uid option.

Inplement the --uid option.

This adds a parser for the --uid argument that fills in the array
of admissible intervals.

In select mode the tables for non-admissible uids are no longer opened.

Therefore we can no longer use the ->table pointer of struct user_info
to find out whether a slot in the hash table is already used. Introduce
the new field ->flags to struct user_info that contains a bit indicating
whether the slot in the hash table is used and another bit that tells
us whether the uid associated with that slot is admissible.