Introduce service_wrapper to avoid direct /etc/init.d calls

While at it drop support for any other syslog daemon than rsyslog.
rsyslog is the package we ship via GRMLBASE anyway.

Known issues:

* /etc/init.d/loadcpufreq depends on systemd-sysv-generator (which
  we disable via grml-live to avoid automatic LSB script startups)
* startx boot option doesn't work yet