sd-daemon: introduce sd_pid_notify() and sd_pid_notifyf()

      sd_pid_notify() operates like sd_notify(), however operates on a
different PID (for example the parent PID of a process).

Make use of this in systemd-notify, so that message are sent from the
PID specified with --pid= rather than the usually shortlived PID of
systemd-notify itself.

This should increase the likelyhood that PID 1 can identify the cgroup
that the notification message was sent from properly.