Debug: fixes for Nuttx.py debug gdb add-ons

also fixed preceding mask calculation in show heaps
Debug: fixes for gdb extension macros
tone_alarm: add PARACHUTE_RELEASE_TUNE
px4io: support PX4IO_P_SETUP_FORCE_SAFETY_OFF

this allows the FMU to force the safety off on the IO board. Useful in
two cases:

 1) vehicles where the safety switch is impractical or not useful
 (eg. HAB planes or internal combustion motors)

 2) doing ESC calibration on multi-copters
pwm: added PWM_SERVO_SET_FORCE_SAFETY_OFF ioctl

this allows the safety switch on px4io to be forced off