pymavlink: raise version
common.xml: added MAV_SYS_STATUS_TERRAIN
common.xml: added MAV_CMD_PREFLIGHT_SET_SENSOR_OFFSETS for 2nd magnetometer
ardupilotmega.xml: merged latest changes from ardupilot git
XML: resolved conflicts between APM and PX4 MAV_CMD_* messages

as discussed with Lorenz and Randy