Prevent LDAP filter returning all users if pam_filter is not set.
Fix default value of $nss_base_passwd variable.

This was broken in 0cb4057595d2 when the $nss_base_passwd and $scope
variables were combined.
Fix nss_base_passwd regex.

It now works fine with or without attributes.
Merge remote-tracking branch 'grahamedgecombe/master'
Merge branch 'fix-nss-base-passwd-regex'
Default to location of Debian LDAP config file.
Merge branch 'debian-config-files'