userdb: move UserDBMatch handling from userdbctl into generic userdb code to allow...

userdb: move UserDBMatch handling from userdbctl into generic userdb code to allow it to be done server side

This moves around the UserDBMatch handling, moves it out of userdbctl
and into generic userdb code, so that it can be passed to the server
side, to allow server side filtering.

This is preparation for one day allowing complex software to do such
filtering server side, and thus reducing the necessary traffic.

Right now no server side actually knows this, hence care is taken to
downgrade to the userdb varlink API as it was in v257 in case the new
options are not understood. This retains compatibility with any
implementation hence.