Bug fixes

This pull request contains two bug fixes:

* If pam_filter was not set in ldap.conf then it used cn=* as the filter (and therefore returning the SSH keys for all users), whereas I believe filtering by uid is the intended behaviour.
* If nss_base_passwd was not set in ldap.conf then the default value is the wrong type and causes the script to fail. (When $nss_base_passwd and $scope were combined and turned into an array in commit 0cb4057595d2 the default value was not updated to reflect the changes.)