Added support for digest-md5 (preliminary). There are three types of digest-md5:
auth, auth-int, and auth-conf. auth is coded and works. auth-int and conf will
follow later, when the service backends are redone, since I'll need to put wrappers
around communications when using those.

Potential for breakage: a digest-md5 session where the server supports more
than one realm. Need to find one that actually does this so I can test for it.