userdbctl: add some basic client-side filtering

userdbctl: add some basic client-side filtering

This adds some basic client-side user/group filtering to "userdbctl":

1. by uid/gid min/max
2. by user "disposition" (i.e. show only regular users with "userdbctl
   user -R")
3. by fuzzy name (i.e. search by substring/levenshtein of user name,
   real name, and other identifiers of the user/group record).

In the long run we also want to support this server side, but let's
start out with doing this client-side, since many backends won't support
server-side filtering anytime soon anyway, so we need it in either case.