BUF_SPACE() was badly buggy, which could lead to memory corruption

this prevents a crash on PX4 if a GPS is attached after startup

put sensor priority above main sketch, to prevent occasional blocking
for sensor data

this is what caused the "no sensors" problem in the 2.69 release

writes will block anyway, which clags up APM

the MTK19 driver supports both the 1.6 and 1.9 protocol

this allows testing of protocol recovery after losing bytes on the GPS
serial link

for choosing what sort of GPS to simulate

the control of altitude between waypoints was broken due to an integer
overflow

this brings px4 into autotest

this will prevent us losing a lot of altitude during the initial
failsafe phase

thanks to Doug for this fix!

this makes the PID library a bit more flexible for smaller range
numbers. Note that this library is used on ArduPlane and Rover, not
Copter