Log the LDAP user used when binding with attrs

It should be possible to see exactly how Sugar is trying to bind
a user with LDAP. Instead of logging, basically, rebinding USERNAME
we log the full bind username including bind attribute and base DN
Added option to check group membership by full DN

By default sugar checks if a user is a mamber of a group just by
checking the bare user name. This adds the option to use the full
DN of the user when checking group membership. This is necessary
because you could have the same username refer to a different
person in a different organization unit.