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 set the snaplen to 1518 (previous default). This approach is more
conservative than the one used in master branch code.

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.
log-pcap: don't limit snaplen.