#2 kill nickname enforcer on ns recover and ns unfreeze

[Christoph Kummer]

If someone doesn't identify for his or her nickname, an
enforcer holds the nickname, so it isn't possible to
change to it anymore. On nickserv recover, the enforcer
is not removed, as one would expect.

also, on nickserv freeze, there is an enforcer if
someone tries to change to the specific nickname, which
isn't killed on ns unfreeze.