dissector: do not panic on unknown hatype

Do not panic on linktypes that are unknown to us. Just skip the usual
dissector entry point and based on user input, print in hex, ascii, or
hex + ascii.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
pcap: fix build error

Various fixes for last commit. Sorry for that.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>