Use $group->getDisplayName() as display name instead of $group->getGID()

[rotdrop]

Currently the display name of a group-member is set to the gid of the group. However, groups also have display-names, so it would be nice to have the display name of a group member set to the display of the group instead of just the gid. GIDs could also be cryptic, depending on the user and group backends which are used.