Credentials

A branch with some credentials cleanup. This was part of the GSSAPI branch, but since it has use, and I'd rather not keep rebasing these changes, I'd like to get them in early.

The rest of the gssapi-auth branch, needs more work:
 * Testing (automated against KDC)
 * Forwarding credentials to other machines.

But you can see how CockpitCredentials in particular evolves to support kerberos:

https://github.com/stefwalter/cockpit/commits/gssapi-auth