ws: Pass headers to CockpitAuth login_finish() virtual method

In our GSSAPI authentication we'll be adding custom headers from
the implementations of this virtual method.
ws: Remove support from CockpitWebServer for handling POST reqs

This is no longer used or needed. Conceptually we should never be
handling POST requests because we have no CSRF protection, or anything like
that anyway.

Anything that changes state should go through thet WebSocket.

Closes #1114
configure: Link appropriate binaries to krb5 libraries
common: Utility function for decoding hex strings

And tests
ws: Implement basic GSSAPI kerberos authentication

The actual authentication happens in cockpit-session, which has
access to the keytab
ws: Implement basic testing of kerberos auth during 'make check'

This requires the 'krb5-server' package or similar to be installed
while running 'make check'