GCS_Mavlink: add MAV_CMD_NAV_VELOCITY

This is an earth frame velocity request message
GCS_MAVLink: generate with new MAV_CMD_NAV_VELOCITY
AP_Mission: support MAV_CMD_NAV_VELOCITY msg
AC_PosControl: add xyz velocity controller
Copter: restore pv_get_horizontal_distance_cm function
Copter: guided mode velocity controller
Copter: enable NAV_GUIDED - do not merge to master