Extend sd_notify interface to support EXTEND_TIMEOUT_USEC

Currently, if knotd startup takes more than 90 seconds (with default settings) service startup fails for start timeout.

It's possible to set TimeoutStartSec to be longer or set async-start in config but more correct way is to tell systemd startup will take more time so it can wait.

There is new EXTEND_TIMEOUT_USEC variable to request more startup time from systemd.

New interface was announced with systemd 236

https://lists.freedesktop.org/archives/systemd-devel/2017-December/039996.html https://www.freedesktop.org/software/systemd/man/sd_notify.html