[Feature request] Add GPSD support

Knowing its own location is useful for MAVproxy on a companion computer as well as when used as GCS. Connecting to a middleware layer for reading GPS data has various advantages such as hardware-independence as well as the possibility of sharing the GPS device among various applications.
To my knowledge, the defacto standard under Linux and most POSIX OSs is GPSD (https://gpsd.gitlab.io/gpsd/index.html). Windows versions or compatible derivatives seem to be available too.
Thus I'd like to suggest a module which talks to GPSD for determining the own position.

Start by reading the linked GPSD documentation and examining MAVProxy's existing module entry points. Define the integration boundary for reading GPSD position data and how the module will expose the vehicle's own location. Done means MAVProxy can obtain its own position through GPSD on the intended Linux or POSIX environments.

Written by the indexing model from the issue text.