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This has been replaced by guided_limit structure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