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>