Add optional systemd(1) support to nsca-ng(8)

Make the server play nicely with systemd(1) (or any compatible init
system) by adding optional support for

- socket activation,
- the "notify" process startup type (see systemd.service(5)),
- the "WatchdogSec" feature (see systemd.service(5)), and
- systemd(1)-style logging to the standard error output (see daemon(7)
  and sd-daemon(3)).

No new nsca-ng(8) option has been added.  When compiled with systemd(1)
support, the server enables these features automatically if it was
socket activated.

The systemd(1) integration will remain completely optional.