Add -u option to groups to filter groups having a user as member

[dbosk]

Right now we have the lsgroups script. But we could get that functionality by adding a -u parameter to the groups command, so that it only outputs the groups containing a certain user.

[dbosk]

This is what canvaslms users -c prgm23 -C Labb | grep "user" also does. However, it's rather slow without caching.