ws: Use private kerberos ccaches

We never want to share kerberos ticket caches with either the root
user (via the kernel keyring or default ccache) or between users
who are both authenticating via cockpit-ws.

 * Prevent use of a default ccache in cockpit-ws
 * Explicitly allocate memory ccache's in cockpit-ws for the
   threads that are authenticating users via libssh
 * Prevent use of a default ccache in cockpit-session

Fixes #1311
What happens in cockpit_creds_push_thread_default_gssapi when this fail?  Will it cope with that cleanly?