sysv-generator: Skip init scripts for existing native services

sysv-generator: Skip init scripts for existing native services

This avoids taking the SysV init script enablement state into account if we
have native units. Otherwise systemctl disable on native unit would not
be respected in the presence of an enabled SysV script.

Also, there's no need to do all the parsing and creation of service files if we
already have a native systemd unit for the processed SysV init script.