Add mod_session support

By setting GssapiUseSessions we enable the module to store a bearer
token with the user and gss names in the client, this way we can allow
clients to perform authentication once but then remain authenticaed
for the duration of the session or until the original credentials expire.

The Secure cookie used to store the token is encrypted using a randomly
generated AES key at process startup. This means multiple apache servers
will not be able to use the same cookie, however the client will reauth
transparently if the cookie cannot be read.