netsniff-ng: Restore tpacket v2 capturing

Some older systems (e.g. RHEL 6) don't have tpacket v3 available, but
only tpacket v2. However, since commit d8cdc6a ("ring: netsniff-ng:
migrate capture only to TPACKET_V3") we solely rely on tpacket v3 for
capturing packets.

This patch restores the possibility to capture using tpacket v2. For now
this is just a fallback if the configure script doesn't detect tpacket
v3 (and thus HAVE_TPACKET3 isn't set). Thus, on most modern systems this
shouldn't change anything and they will continue using tpacket v3.

For now this fix contains quite a bit of ugly #ifdefery which should be
cleaned up in the future.

Fixes #76

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
dissector: check for correct netlink linktype

It's supposed to be LINKTYPE_NETLINK, not AF_NETLINK as otherwise
the pkt_type fixup cannot be done correctly.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>