networkd: add bus property exposing network namepace ID we run in

networkd: add bus property exposing network namepace ID we run in

This is useful for clients to determine whether they are running in the
same network namespace as networkd.

Note that access to /proc/$PID/ns/ is restricted and only permitted to
equally privileged programs. This new bus property is primarily a way to
work around this, so that unprivileged clients can determine the
networkd netns, too.