pcap: set snaplen to MTU if available.

Main objective of this patch is to use a dynamic snaplen to avoid
to truncate packet at the currently fixed snaplen.

It set snaplen to MTU length if the MTU can be retrieved. If not, it
does not set the snaplen which results in using a 65535 snaplen.

libpcap is trying to use mmaped capture and setup the ring by using buffer_size
as the total memory. It also use "rounded" snaplen as frame size. So if we set
snaplen to MTU when available we are optimal regarding the building of the ring.
pcap: add 'promisc' YAML configuration variable

This patch adds a promisc variable to pcap configuration. It is
used to decided if interface is switched to promiscuous mode.
log-pcap: don't limit snaplen.
pfring: delete unused define.
pcap: add snaplen YAML variable

This patch introduces 'snaplen' a new YAML variable in the pcap section.
It can be set per-interface to force pcap capture snaplen. If not set
it defaults to interface MTU if MTU can be known via a ioctl call and to
full capture if not.