This fixes a bug which did not show up on FreeBSD, but on MacOS.

Use the portable pcap_set_immediate_mode() function instead of
BIOCIMMEDIATE.

This patch makes sure that all heaaders start at a four byte
boundary. This is done by not having the ethernet header, which
is 14 bytes long, in the packet buffer.
This fixes the remote mode for platforms having strict alignment
restrictions, like ARM. Without this patch you can't use the remote
mode on a Rasperry Pi running FreeBSD.
This patch has been tested on Linux (Intel) in local and remote mode
and one FreeBSD (Intel and ARM) in local and remote mode.

Signed-off-by: Michael Tuexen <tuexen@fh-muenster.de>

When using libpcap, the function packet_socket_receive was ignoring the
direction. If there were inbound packets sniffed the following try to
read the packet from the tun descriptor blocked.
This patch makes packet_socket_receive aware of the direction and therefore
the libpcap implementation has the same functionality as the Linux one.
It just uses two pcap handles, one for each direction.

Tested on FreeBSD.

Signed-off-by: Michael Tuexen <tuexen@fh-muenster.de>

Initial commit for the packetdrill network stack testing tool:

  https://code.google.com/p/packetdrill/



Signed-off-by: Neal Cardwell <ncardwell@google.com>