AHRS/EKF: how to handle loss of absolute position sensor data

From @priseborough in https://github.com/ArduPilot/ardupilot/pull/6026#issuecomment-294379550 (and related to #5846):

> @OXINARF, @rmackay9 and @tridge A fallback to relative position mode is something that has implications for the controllers as some rely on an absolute position. Currently the EKF, once it has set a WGS-84 origin, will continue to dead-reckon and stay in absolute position mode, outputting a WGS-84 position estimate if GPS is lost, provided it has another sensor that it can use to dead-reckon.
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> This is required because the exisiting AHRS interface only allows a binary healthy/unhealthy status for position. In practice, different vehicle types and different operating environments require different levels of accuracy, making it impossible for the EKF to arbitrarily decide when the position uncertainty due to dead-reckoning is no longer adequate for navigation.
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> My recommendation is that we move to an interface where the AHRS and EKF provide a 1-std estimate of the position and velocity error, and the controllers decide individually whether that meets the navigation accuracy requirements and whether a navigation mode fallback is required.