tls: set verify function on session, not credentials
tls: set verify function on session, not credentials

gnutls_session_set_verify_function() was added in 3.4.6 (which we depend
on) and it makes an awful lot more sense to set it here directly instead
of indirectly on the server credentials.  The verification function is
about the *peer* credentials, and is absolutely tied to the session (cf.
the callback pointer type).

This will also make it easier to clean up our handling of the server
certificate in commits to come.

mvollmer/cockpit,garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,garrett/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,garrett/cockpit