shutdownd: rework interface, allow subscribing to scheduled shutdowns

shutdownd: rework interface, allow subscribing to scheduled shutdowns

This extends the shutdownd interface to expose schedule shutdown
information in /run/systemd/shutdown/schedule.

This also cleans up the shutdownd protocol and documents it in a header
file sd-shutdown.h.

This is supposed to be used by client code that wants to control and
monitor scheduled shutdown.