add a 'AuthDBMType' so that users can select what type of DBM they want to

use to authenticate users.
I think this removes a 'showstopper' (deprecating mod_auth_db)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92616 13f79535-47bb-0310-9956-ffa450edef68