netdev-linux: Don't restrict policing to IPv4 and don't call "tc".

netdev-linux: Don't restrict policing to IPv4 and don't call "tc".

Mike Bursell pointed out that our policer only works on IPv4
traffic--and specifically not IPv6.  By using the "basic" filter, we can
enforce policing on all traffic for a particular interface.

Jamal Hadi Salim pointed out that calling "tc" directly with system() is
pretty ugly.  This commit switches our remaining "tc" calls to directly
sending the appropriate netlink messages.

Suggested-by: Mike Bursell <mike.bursell@citrix.com>
Suggested-by: Jamal Hadi Salim <hadi@cyberus.ca>