Plane : navigation.pde : Modified groundspeed_undershoot calculation to use speed component in fwd direction

The previous calculation of groundspeed_undershoot could leave an aircraft flying backwards uncorrected if the backwards velocity was high enough
Merge branch 'master' of git://github.com/diydrones/ardupilot

# By Randy Mackay (8) and Andrew Tridgell (7)
# Via Randy Mackay
* 'master' of git://github.com/diydrones/ardupilot:
  Copter: correct fence parameter
  Copter: integrate AC_Fence in place of limits
  AC_Fence: first implemenation of library
  AutoTest: reduce copter turn rate in fly square
  Rover: add I2C error count to PM message
  Plane: add I2C error count to PM message
  Copter: add I2C error count to PM message
  Plane: support 12 output channels on PX4
  Plane: if airspeed enabled, require 5m/s to unsuppress throttle
  Copter: support all 12 channels on PX4
  RC_Channel: support 8 auxillary RC channels
  HAL_PX4: setup for 12 output channels
  HAL_PX4: support 12 output channels with PX4IO
  AP_HAL: define RC_12
  Copter: Leonard's fix for trapezoidal frame