varlink: add new io.systemd.Job interface

varlink: add new io.systemd.Job interface

This commit creates a new varlink-io.systemd.Job.c file and puts
the job related varlink types into this file. Those will be
used by the upcoming io.systemd.Unit.StartTransient.

Note that the property names follow the D-Bus naming to make a
future "systemctl show" transition from D-Bus -> varlink easier.

Thanks to @ikruglov for suggesting this preparation commit and
to Lennart for suggesting the D-Bus compatibility considerations.