man: netsniff-ng: add note about multithreading

Add a paragraph about multi-threading with netsniff-ng.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
make: minor cleanups in Makefile

Remove some unneeded stuff, and move other things around.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
mausezahn: show long version on cli

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