Capture len 1.3 v1.1

Reduced patchset compared to the one proposed to fix #680 on master.

I've assumed the people using 1.3.x have working configuration so I've modified the logic of the "pcap: set snaplen to MTU if available.". It set snaplen to MTU length if the MTU can be retrieved. If not, it does set the snaplen to 1518 (previous default).