Bump the default MTU to 8000 bytes
Bump the default MTU to 8000 bytes

This is the MTU on the local virtual ethernet link, not the "safe
outgoing MTU" which is used for datagram forwarding.

We would like the MTU on this link to be large to reduce the processing
overhead per byte of payload, however the maximum safe read and write
size on Hyper-V sockets seems to be 8192; therefore pick a round number
< 8192.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit