Copter: add nav_guided suport to Auto mode
Copter: accept guided pos updates when doing NAV_GUIDED cmd
Copter: accept condition-yaw commands in guided
GCS_Common: minor typo in comments
GCS_Mavlink: add MAV_CMD_NAV_VELOCITY

This is an earth frame velocity request message
GCS_MAVLink: generate after adding CMD_NAV_VELOCITY
AP_Mission: support MAV_CMD_NAV_VELOCITY msg
AC_PosControl: add xyz velocity controller

Velocity controller interpretsthe velocity requests as
desired velocities (i.e. feed forward).  These are then used to update
the target position and also added to the target velocity.
Also renamed the set_desired_velocity() function to
set_desired_velocity_xy() to make clear only lat and lon axis are
updated.
Copter: restore pv_get_horizontal_distance_cm function
Copter: guided mode velocity controller
AC_WPNav: integrate set_desired_velocity_xy function name change
Copter: guided mode sets desired velocity instead of target velocity
Copter: MAV_CMD_CONDITION_YAW accepted outside missions
Copter: DO_SET_ROI accepted outside missions

Moved ROI logic to new set_auto_yaw_roi function.
AC_WPNav: update yaw only when track is at least 2m
Copter: guided reset yaw only when initialised
Copter: Add parameter LAND_REPOSITION to enable/disable user input during auto-mode landings and descents
Copter: allow disarm in autotune mode
Copter: allow auto-disarm in autotune mode
Copter: Wait 4 seconds before beginning to land during failsafes