Split out common code for deleting users

[wens]

Currently there are at least two code paths that can delete users:

1. When a new user registers, the register system will look for an empty slot, and if one cannot be found, it will proceed to "expire" an old user.
2. The reaper tool under util, which "expires" users wholesale.

The user deletion code is written into these code paths, without any sharing. Over time, as the reaper tool didn't get much usage, the differences grew.

The idea is to move common code for user deletion under common/bbs.