pam-systemd: talk to logind via varlink

pam-systemd: talk to logind via varlink

This makes sure we now use Varlink per default as transport for
allocating sessions.

This reduces the time it takes to do one run0 cycle by roughly ~10% on my
completely synthetic test setup (assuming the target user's service
manager is already started)

The D-Bus codepaths are kept in place for two reasons:
* To make upgrades easy
* If the user actually sets resource properties on the PAM session we
  fall back to the D-Bus codepaths, as we currently have no way to
  encode the scope properties in JSON, this is only supported for D-Bus
  serialization.

The latter should be revisited once it is possible to allocate a scope
unit from PID1 via varlink.