Port last(1) to use utmpx.

Port last(1) to use utmpx.

Basically there are three major things I changed about last(1):

- It should use ut_type instead of determining by hand what type of
  record was given.
- It should now keep track of ut_id's instead of TTYs. This means the
  ttylist has been renamed to the idlist, storing all the ut_id's it has
  processed until the next reboot.
- I've removed the signal handler. Because our wtmp is rotated so often,
  it makes little sense. Even on a simple piece of hardware it should be
  capable of grinding through megabytes of logs in a second.