netsniff-ng: also capture if NIC is currently down

There is actually no reason why netsniff-ng should fail if the NIC is
down at startup. We still can setup everything and already capture at
the time it goes up. This might be useful when replugging cables on
servers, for instance.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
man: man page updates for bpfc, curvetun

These are the writings so far. For sure they are not yet finished, so
I will push updates for both of them as soon as possible, but for now
this is a first version people can start to read.

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