AP_RangeFinder: auto-update PX4 ll40ls max/min distance

this allows the range of the Lidar to be set by the user using
RNGFND_MAX_CM and RNGFND_MIN_CM
Copter: enable Guided velocity controller for Pixhawk
Copter: remove unused pilot_yaw_override flag
Mission: support GUIDED_ENABLE and GUIDED_LIMITS

This replaces the ardupilot only NAV_GUIDED command.
Also remove support for NAV_VELOCITY mission command which will be
replaced by SET_POSITION_TARGET non-mission command.
Copter: support GUIDED_ENABLE and GUIDED_LIMITS

Split of NAV_GUIDED into these two command necessitated guided mode
store the limits
Copter: remove nav_guided structure

This has been replaced by guided_limit structure
Copter: support SET_POSITION_TARGET messages
Copter: accept velocity requests in Auto-Guided mode
Copter: minor param description updates
Baro: minor param description updates
BattMon: minor param description update
BattMon: minor param description additions
AC_Motors: param description addition
Merge remote-tracking branch 'arducopter/master'

Conflicts:
	ArduCopter/APM_Config.h