New Digest user file syntax in Apache 2.2

Hey Jamie,

Apache 2.2 uses new syntax for Digest authentication sources.

It changes from:

AuthDigestFile /path/to/davusers
AuthDigestGroupFile /path/to/davgroups

To:

AuthDigestProvider file
AuthUserFile /path/to/davusers
AuthGroupFile /path/to/davgroups

This effects our users on Fedora Core 5, and will probably effect some of the other Operating Systems as they adopt Apache version 2.2.

Note that AuthDigestProvider can offer other sources--like a dbm file. We won't be using that feature, but probably worth including in the Webmin module changes.