send: propagate DSCP bits to the outer tunnel

send: propagate DSCP bits to the outer tunnel

Like many, I am using WiFi a lot and often on congested networks.
 Without this, Wireguard strips the DSCP bits, preventing WME from
 kicking in and improving the audio/video experience.

Yes, it's technically an information leak. Who cares? It's not like if
traffic analysis based on packet sizes or timings wasn't a thing.

This is the first patch of the serie, more work has to happen on ECN

Signed-off-by: Florent Daigniere <nextgens@freenetproject.org>