only list uids from the actual user uid range

Summary:
the max uid is now actually being used to cap the displayed uids (e.g.
I have a system user created beyond my MAX_UID of 60000 that were
erroneously shown because the detected maxuid was unused)

Test Plan: the excess user no longer shows up

Reviewers: ngraham

Reviewed By: ngraham

Differential Revision: https://phabricator.kde.org/D26964