Replace hardcoded krb5.conf with gssproxy

Instead of having our own krb5.conf and pulling in the kerberos ccache from the user environemnt inside the flatpak runtime, let's use gssproxy instead. GSS-Proxy's proxymech.so plugin intercepts GSSAPI calls and forwards them to the gssproxy service. By bind-mounting the gssproxy service socket in the flatpak environment we outsource dealing with all the kerberos configuration and ticket management to the host user session.

As a bonus we avoid exposing the kerberos tickets to the flatpak environment.