Failsafe: make the safety margin 100 us

This is a bit tight on the range of trims people might use
so I don't really like it.  I think we discussed applying
this only to throttle.
ManualControl: remove unused defines
manualcontrol: always validate flightmode channel when it is configured

Previous code would skip validating the flightmode when FlightModeNumber
was set to 1.  Whenever a flightmode channel is configured, it should
be validated so that it can consistently be used to detect a disconnected
transmitter.

Signed-off-by: James Cotton <peabody124@gmail.com>
Transmitter: allow channel specific valid input ranges

Now throttle uses 100 us to allow Frksy users to get a legitimate failsafe
and other channels remain at 250 microseconds.
Transmitter: separate checking flight mode configuration and validity

Checking for a valid flight mode input needs to run through the hysteresis
loop with the other inputs.  The previous commit had added it but put it in
the configuration loop which means any glitches would instantly trigger
failsafe rather than wait 100 ms.
Merge pull request #644 from peabody124/failsafe_window

Failsafe: make the safety margin 100 us