Modify type for the raw link layer

[zecke]

According to http://www.tcpdump.org/linktypes.html the value is 101
but in reality it is 12 (and on OpenBSD 14). While browsing through
the list on OpenBSD the DLT_LOOP is 12 (and not 108). In the long it
might make sense to back these values with cgo (and pcap/bpf.h).

[zecke]

I don't know your taste. I can start using pcap.DLT_RAW and pcap.DLT_RAW (which in turn would return C.DLT_RAW) to make this work universally. The reason I didn't do it yet is the question if you see a hierarchy between pcap.go and enums.go (e.g. if enums.go should depend on elements from pcap.go) or if enums.go should replicate the ldflags/cflags and import "C" directly as well