arpwatch 2.1a15: Cleanups for arpwatch:

arpwatch 2.1a15: Cleanups for arpwatch:
- mark configuration so they don't get wiped on upgrade
- don't pass -p, for nopromisc, as an option since that isn't
  supported (must be a debian patch which is not included in OE)
- remove RUNAS which was used to specify a non-root user to run as
  since it isn't support (must be a debian patch which is not
  included in OE)
- Use volatiles to create required /var directory.
- Inherit update-rc.d so it is started and stopped during
  uninstall, upgrade and uninstall.