ldap: Several fixes and improvements
* changed ldap_installer::install() to activate() so that ldap can be
enabled again after failing the first time
* if some information is not available from ldap server return
"better" values (e.g. no group instead of group = NULL)
* added function Ldap_User::loaded() which is called by user profile
* added configurable admin email address if not given by server
* make ldap work when there is no group management on ldap server
(create local groups for everybody and registered_users) and allow
guest user to be managed by server (configurable guest name)
* require everybody_group and registered_users_group to be in groups
The changes regarding renaming install() to activate() could be superfluous with the other fixes since there should not go anything wrong during install and uninstall. However, they do no harm and are really practical if something does go wrong.
Hope you can make some use of those (I diffed against current git checkouts).
Fixes#1834.
