Add --with-fpm-systemd option to report health to systemd, and
systemd_interval option to configure this. The service can now use
Type=notify in the systemd unit file.

Systemd status line will looks like:
Status: "Processes active: 0, idle: 5, Requests: 90, slow: 0, Traffic: 10req/sec"

Watchdog mode is also supported. In this case watchdog interval,
configured in the unit file, override the systemd_interval option.

When not used (default), no change from previous version.