Use constants for PWM ranges instead of the magic numbers 0, 1000 and
2000.
Provide temporary support for old clients that try and send a channel
index instead of a servo middle when using MSP_SET_SERVO_CONF.

Old GUI clients can still be used to SET channel forwarding, but cannot
be used to READ the status of channel forwarding until they are updated
to use MSP_CHANNEL_FORWARDING/MSP_SET_CHANNEL_FORWARDING.

To the user it will mean their GUI may not match the stored
configuration.  However there is no way to DISABLE channel forwarding
other than a reset if they have enabled it using an old client.

This situation is not ideal, but without this commit it would mean that
bad data could be stored and used incorrectly by baseflight if a user
enables channel forwarding using an old client.