Avoid warnings from nwfilter driver when run non-root

Avoid warnings from nwfilter driver when run non-root

When run non-root the nwfilter driver logs error messages about
being unable to find iptables/ebtables commands (they are in
/sbin which isn't in $PATH). The nwfilter driver can't ever work
as non-root, so simply skip it entirely thus avoiding the error
messages

* src/conf/nwfilter_conf.h, src/nwfilter/nwfilter_driver.c,
  src/nwfilter/nwfilter_gentech_driver.c,
  src/nwfilter/nwfilter_gentech_driver.h: Pass 'bool privileged'
  flag down to final driver impl
* src/nwfilter/nwfilter_ebiptables_driver.c: Skip initialization
  if not privileged