This prevent any parsing ambiguity and also allow to easily expand
the data saved by simply adding more fields when packing the data.

If a client, by its own initiative, decides to try to reset the
credential status by sending an Authorization header, let's oblige
and kill the current authorization context.

In a connection oriented case we kill the GSS context, and if
sessions are in use we set an expired, NULL session, so that the
client will be effectively logged out unless a complete authentication
is performed again.

Closes #16

This reverts commit e9c92795d87a316ea47f6bf37c9636e86eec57e7.

AESGCM is a neat idea but it is not really appropriate to be used in
mod_auth_gssapi because we cannot gurantee that the nonce will never be
reused. It is not very probable, and it is also not easy to force the
server to generate so many encyrpted sessions to have a good chance of
a collision that I know of, but better to avoid the whole issue, than
risk unforseen cases where it may happen.

On success do not forget to send the last negotiate packet (if any)
to the client within the 200 Reply.

Fixes #21

When doing fallback basic auth, we may also want to honor the
configured directive about storing delegated credentials.
Detect if we are configured to store them and set the appopriate
init_sec_context flag that will cause the accept_sec_context call
to get valid delegated credentials for later storage.

In some cases (like during directory listing) Apache will re-run the
authentication code. Many GSSAPI mechanism have replay detection so
we cannot simply rerun the accept_sec_context phase. Others require
multiple steps. When authntication has already been estalished just
implicitly consider the authentication successfully performed and
copy the user name. Otherwise fail.
If a subrequest hits a location with a different mod_auth_gssapi
configuration warn but do not error off right away.

Fixes #15

The principla name is used as a file name, any embedded path separators
are going to cause trouble if used in the file name, so we need to escape
them away. Usee ~ as the escape chracter (~~ to escape ~ itself)

Fixes #14