Fix servo middle/channel forwarding variable re-use.  Adds a new
capability flag which is returned by MSP IDENT (CAP_CHANNEL_FORWARDING).
Adds two new MSP commands that are specifically for configuring channel
forwarding (MSP_CHANNEL_FORWARDING/MSP_SET_CHANNEL_FORWARDING).  Servo
'middle' and 'forward from channel' are now stored independently, as
they probably should always have been.