quot: Add tests

To facilitate the task, we change the comparison function so that users
with equal filesystem usage are sorted by UID, and add an undocumented
option that prevents quot from replacing numerical UIDs with names. We
also switch from getfsfile(3) to getmntpoint(3) so the first line is
identical regardless of whether we pass quot a mountpoint or a device.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53133

(cherry picked from commit aa870a1935bccb66e02c4c31630706768a3e7d74)
(cherry picked from commit b5f3f1a661ab69986c3379fa4c9c2b38785933db)
(cherry picked from commit b41551753a2baff6646e2958acb4a7fcd205b69c)