changed sd-notify api

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
documented the module

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
made sd-notify behavior saner

Instead of using a multiple symbols to communicate result, do nothing
when not being run under systemd and fail hard on other issues.

Signed-off-by: Jan Dvořák <mordae@anilinux.org>