netsniff-ng: enable jumbo frames on default in tpacket_v3

Since frames are stored more compressed and contiguous, we can also
enable jumbo support in pcap dumps by default, since we have no further
restrictions in terms of ring buffer frame size.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
man: netsniff-ng: update man page to document when -J is needed

--jumbo-support is now not always needed anymore. So document when
one needs it and when not.

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