Add ability to use multiple providers for basic and digest authentication.

The syntax is:

AuthBasicProvider file dbm
AuthUserFile conf/basic-user
AuthDBMUserFile conf/basic-user-dbm


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@96869 13f79535-47bb-0310-9956-ffa450edef68