[Feature Request] GPSd support

[VA2XJM]

I'd like to make a mobile ADSB/Mode S receiver. It would allow to track while on the move (in movement or just parked somewhere). It gives flexibility without the need to shutdown mlat-client, find out new coordinates and restart it manually.

I've checked the code of the client and it is beyond my skills but I think I get what changes is needed.

1. add a flag for GPSd (--gpsd) that override --lat --lon.
2. add a bit of scripting to retrieve GPS position every 1 second
3. Add GPSd coordinates with every message sent to the server.
4. mod the server to use transmitted coordinates with every message.

I have the hardware ready to test, I just lack those python ninja skills...