[BUGFIX] Escape users DN when filtering groups

To find the groups of a user by filtering the groups where the
authenticated user is in the members list some characters
in the users distinguished name can make the filter expression
invalid.

At the moment this problem was experienced with brackets which
are also used in the filter, e.g.:
(&(objectClass=group)(member={USERDN}))

Therefore any brackes in the users DN will be escaped with backslashes
when filtering the groups the user is member in.