User Lat/Lon setting

Based on initial code supplied by VK1ET

Allow the user to specify their receivers (technically their aerials)
physical location. This is required for decoding ground positions, and
also for uploading aircraft data to various sharer sites.

The position can be hardwired by setting the MODES_USER_LATITUDE_DFLT
and MODES_USER_LONGITUDE_DFLT constants before compilation, or entered
at runtime via the new command line switches --lat and --lon.   Either
way, the values are checked for validity before use.