HAL_PX4: make PX4 snprintf support %S format

just maps it to %s for normal printf
SITL: support vsnprintf with %S on SITL
AP_InsertialSensor: added support for MAVLink user interaction

allows APM to ask user to print enter to continue via MAVLink messages
during accel calibration
AP_InertialSensor: show calibration offsets for success and failure

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
Copter: use new accel cal interact over MAVLink
Plane: support 3D accel calibration over MAVLink
AC_Fence: fixed example build
AC_WPNav: fixed example build
AP_Baro: fixed example build
AP_InertialNav: fixed example build
AP_InserialSensor: fixed example build
DataFlash: fixed example build