Add support for reporting status via sd_notify()

Add support for reporting status via sd_notify()

sd_notify() may be called by a service to notify the service manager
about state changes. It can be used to send arbitrary information,
encoded in an environment-block-like string. Most importantly, it can be
used for start-up completion notification.

Add libsystemd check to autoconf script and when the library is detected
add calls to sd_notify() around the server->reload_status changes.

Co-authored-by: Petr Špaček <pspacek@isc.org>