Increase user's hostname field (aka user_t.comp) from 30 to 60 chars

Many dynamically-generated (e.g residential) hostnames are much > 30
chars. And we can now increase user.tab fields fairly arbitrarily, so
let's. :-)

Also rename LEN_COMP to LEN_HOST. Some day I'll change all the user_t
comp references to 'host', but not today.