pidref: add explicit concept of "remote" PidRef

pidref: add explicit concept of "remote" PidRef

This PidRef just track some data, but cannot be used for any active
operation.

Background: for https://github.com/systemd/systemd/pull/34703 it makes
sense to track explicitly if some PidRef is not a local one, so that we
never attempt to for example "kill a remote process" and thus
acccidentally hit the wrong process (i.e. a local one by the same PID).