Move handling of sysv initscripts to a generator

Reuses logic from service.c and the rc-local generator.

Note that this drops reading of chkconfig entirely. It also drops reading
runlevels from the LSB headers. The runlevels were only used to check for
runlevels outside of the normal 1-5 range and then add special dependencies
and settings. Special runlevels were dropped in the past so it seemed to be
unused code.

The generator does not know about non-generated units with a value set with
SysVStartPriority=. These are therefor not taken into account when converting
start priority to before/after.