Plumbing to perform SELinux checks in varlink API (#38146)

This PR does minimal changes to introduce varlink support. Ideally, the
code should switch to using `mac_selinux_get_our_label()` and new
`mac_selinux_get_peer_label()`. But I leave it for now to minimize
breakage. `mac_selinux_get_peer_label()` remains unused.

This is a prep step to merge
https://github.com/systemd/systemd/pull/38032