This adds the functionality to run a user search with a filter for LDAP
authentication. The search is done after successful bind, as the binding
user. The typical use case is to limit authentication to users who are
member of a group or under a certain OU. For example, to only match
users in the "Syncthing" group in otherwise default Active Directory
set up for example.com:
<searchBaseDN>CN=Users,DC=example,DC=com</searchBaseDN>
<searchFilter>(&(sAMAccountName=%s)(memberOf=CN=Syncthing,CN=Users,DC=example,DC=com))</searchFilter>
The search filter is an "and" of two criteria (with the ampersand being
XML quoted),
- "(sAMAccountName=%s)" matches the user logging in
- "(memberOf=CN=Syncthing,CN=Users,DC=example,DC=com)" matches members
of the group in question.
Authentication will only proceed if the search filter matches precisely
one user.
