Rename `mtu` to `safe_outgoing_mtu`
Rename `mtu` to `safe_outgoing_mtu`

There are currently 2 important uses of `mtu` in the current code:

1. the maximum UDP datagram size we promise to send without fragmentation.
   <-- this has been renamed `safe_outgoing_mtu`
2. the MTU on the virtual ethernet link to the VM. This is a link-local
   concern since we proxy the TCP streams.

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

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit