configure: Only link against libpcap on FreeBSD.

On other platforms there is no benefit to linking against libpcap, because
it is not used.

Signed-off-by: Ben Pfaff <blp@nicira.com>
CC: Ed Maste <emaste@freebsd.org>
mpls: Allow l3 and l4 actions to prior to a push_mpls action

* Update the order in which actions are committed and thus
  appear in the datapath such that MPLS actions appear after
  l3 and l4 (nw and port) actions.

  In the case where an mpls_push action is present it should ensure
  that l3 and l4 actions occur first, which seems logical as
  once a mpls_push has occur the frame will be MPLS rather
  than IPv4 or IPv6.

  In the case where there is an mpls_pop action is present this should
  not make any difference as the frame will have been MPLS to start with
  and thus not satisfy the pre-requisites for  l3 or l4 actions.

* Update commit_set_nw_action() to use the base ethertype when considering
  eligibility to commit l3 (nw) actions. This allows l3 actions to be
  applied so long as the frame was originally IPv4 or IPv6, even if
  an mpls_push action will be applied and thus flow indicates the
  frame will be MPLS.

* Make actions that may modify port or network information conditional on
  the flow's ethernet type being an IP ethernet type. This is to ensure
  that actions that modify network and port information do not occur
  on non IP packets, for example if an mpls_push action has changed a
  packet from IP to MPLS.

  Note that modification of network data is already prevented by
  virtue of commit_set_nw_action() only having cases for when the
  ethernet type of the flow is  IPV4 or IPV6. The conditionality
  of network actions on the ethernet type has been added to
  do_xlate_actions() to make it explicit.

* Add a check to commit_set_port_action() to ensure that the base
  flow is IP. This protects against the case where move_reg is used
  to change transport ports after an MPLS header is pushed.

Signed-off-by: Simon Horman <horms@verge.net.au>
[jesse: Add check for an IP protocol when committing L4 actions.]
Signed-off-by: Jesse Gross <jesse@nicira.com>