LDAP: Force gid 0/99999 for everybody/registered users groups
LDAP: Bugfix lookup_group() and lookup_group_by_name(): use index 0 from cn-attribute
LDAP: use == to compare values, = assigns new value
LDAP: Bugfix, use correct name of LDAP_EVERYBODY_ID constant
LDAP: in ldap_search() request only interesting attributes

- We had an issue that some users could not login. Reason was, that these users
  had a large 'jpegPhoto', which was (supposedly) to big for the session
  variable.