switch from direct /etc/init.d/service invocations to service(8)

/usr/sbin/service abstracts the interaction with system dæmons
independent of the chosen init system. This is important because
invoking sysv initscripts directly is problematic when running under
systemd (or upstart) dæmon supervision.

sysvinit:
	/etc/init.d/<daemon> {start | stop | restart}

systemd:
	systemctl {start | stop | restart} <daemon>.service

upstart:
	initctl {start | stop | restart} <daemon>

generic init abstraction provided by sysvinit-utils on Debian, valid
for all supported init systems listed above:
	service {start | stop | restart} <daemon>

While we should probably switch to documenting systemctl directly, once
the default has been changed in sid officially, it's better to document
generic abstractions like service or update-rc.d for the time being.

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>