Allow libpcap to read files with some additional link-layer type values

Allow libpcap to read files with some additional link-layer type values

patch has been contributed by Guy Harris from libpcap/tcpdump. It
resolves collision between DLT_* values on various OSes. The issue
prevents correct interpretation of link layer information in capture
files which might come from another OS. To resolve this libpcap/tcpdump
community introduced a LINKTYPE_* values. The patch provides translation
between DLT_* and LINKTYPE_* for OpenBSD. More details can be found
here: https://www.tcpdump.org/linktypes.html

No objection from OpenBSD community.

OK sthen@