When studying the Linux source code and Wireshark dumps it seems like
the no_frag flag in the IPv4 header is always set. Following discussions
in the Internet on this subject indicates that modern routers never
fragment packets, and that it isn't even supported in many cases.
Adding to this that incoming messages forwarded on the tap interface
never even pass through a router it seems safe to always set this flag.
This makes the IPv4 headers of forwarded messages identical to those
sent by the external sockets, something we must consider desirable.
Signed-off-by: Jon Maloy