Factor common code out of add() and new() into nat_reset()

Factor common code out of add() and new() into nat_reset()

The common nation wipe code is not quite identical, and it doesn't
wipe the nation thoroughly enough.  The new code does.

Changes to both commands:

* Wipe nat_update, nat_ann, nat_access, and nat_contact.  Bug: should
  set nat_ann to the number of announcements.

Changes to add command:

* Don't wipe for status active and god.  Before, nat_relate and
  nat_flags where wiped then.

Changes to newcap command:

* Wipe nat_hostaddr, nat_hostname, nat_userid, nat_dayno, nat_minused,
  nat_reserve, nat_last_login, nat_last_logout, nat_newstim,
  nat_annotim, nat_relate, nat_rejects, nat_flags.