RTL fix so that if it starts rtl-ing from above 80m it returns home
while descending instead of descending at initial position.
add get and set_target_alt_for_reporting
Failsafe events changed to errors so they are more obvious.
Errors recorded to dataflash for failure to init compass and optical flow sensor.
Errors recorded for pwm failure.
Resolved a compile error when dataflash logging is disabled.
Consolidated RTL state to be captured by rtl_state variable.
Combined update_RTL_Nav and verify_RTL functions which performed the same function but one was for missions, the other for the RTL flight mode.
Renamed some RTL parameters and global variables to have RTL at the front.
Landing detector now checks accel-throttle's I term and/or a very low throttle value
Randy Mackay
Andrew Tridgell
Jason Short