mainloop: Add support for systemd notifications

When compiling with systemd support, notify systemd about our status
and readiness via sd_notify().

Also set the service type to Type=notify in the systemd unit file.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
mainloop: refactor systemd related calls to thin wrappers
mainloop: Add support for clearing the status

This introduces a new function, main_loop_clear_status(), that will
clear the status when running with systemd support enabled. It also
changes the code that under normal conditions, no status is shown.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>