GCS_MAVLink: merged latest changes from upstream
AP_Mission: removed use of MAV_CMD_NAV_VELOCITY and MAV_CMD_NAV_GUIDED
Copter: removed use of removed MAV_CMD_NAV_ commands
XML: resolved conflicts in MAV_CMD_ enum

this moves a couple of commands to be MAV_CMD_DO_ commands, after
discussions with Lorenz and Randy
GCS_MAVLink: rebuild MAVLink headers