Add parameter to inhibit failsafe in AUTO mode

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For some AUTO missions, failsafe needs to be disabled. When the mission 
prohibits heading towards home from any point (could hit a mountain) or is 
expected to fly beyond reliable control range.  Even momentary RTL can be 
disruptive for aerial photo mission.

If failsafe is completely disabled, then we lose a good safety feature before 
and after the autonomous mission.

Please add:

if MODE=AUTO  and FS_AUTO  = "1" - then  let failsafe condition use setting in 
FS_SHORT_ACTN  or FS_LONG_ACTN

if MODE=AUTO and FS_AUTO = "0" - then resume mission and inhibit use of 
failsafe actions

Original issue reported on code.google.com by andre.kj...@gmail.com on 2 Jun 2013 at 9:47