(add): Initialize nat_last_login and nat_last_logout to zero rather

(add): Initialize nat_last_login and nat_last_logout to zero rather
than current time.  Nation addition time is irrelevant.
(natstr): Document zero nat_last_login, nat_last_login mean never
logged in/out.
(player_main): Test nat_last_login zero instead of nat_hostaddr empty.

(player_main): Assign nat_userid, nat_hostname, nat_hostaddr straight
from player.  The last connection's host is now shown as dotted quads
instead of nowhere when the name isn't known.
(natstr): Document that nat_userid, nat_hostname may be empty.

(player_main): Simplify printing last connection address: nat_hostaddr
can't be empty here; don't bother to substitute "nobody" for empty
user.