Make action=query&list=users use User::getRights()

Make action=query&list=users use User::getRights()

Made action=query&list=users use User::getRights() if
usprop rights given. This not only removes redundant
code, but makes it execute the UserGetRights hook, so
that this now includes rights given by Extensions (eg.
CentralAuth does that).

Patch Set 2: Modified the User class to be able to
inject further data into User::newFromRow() and using
that to inject the groups taken out of one SQL query
(for performance reasons). Furthermore I've split up
the query in ApiQueryUsers.php into one for user data
and one for the groups, to only have one row for each
user.
After all the perfomance of this should now be ok, not
extremly good, but bearable (though I couldn't test it
deeply, as I don't have much data in my CentralAuth
environment).

Change-Id: Ie5b2924abb82ac254c77e1d04cc4d5b308962dad