user.py

Previously Unix UIDs/GIDs were allocated by using the highest used ID + 1.
This caused ID reuse when the newest user/group was deleted. In addition, the
implementation did not work on MariaDB (at all, it was not possible to create
users/groups).

The new approach accounts for all IDs ever used regardless of whether or not
users/groups are deleted. It always allocates the lowest ID in the configured
range that was never used.

Aside from the different allocation algorithm, this change introduces a
generic locking mechanism and prerequisites for testing migration scripts.