More advanced transponder database fetching options

[bjorgan]

#49 will give a basic implementation of transponder database fetching, but will overwrite the user's existing database by writing directly to the assumed DB path. It should rather merge, compare entries, give option to force overwriting, ensure that everything is written to the directory as defined internally in flyby, give option for trying to write to system-wide directory, etc.

This can be done by implementing extra command line options similar to flyby's --update-tle-db, to be applied to a fetched database file saved to a temporary file.

Will probably be more complex, add additional transponder database CLI utility.

• Switch from uplink etc arrays to transponder structs
• Take transponder database file(s) as input, write to database structure
• Merge with existing database, wait for user response if there is a difference for an entry
• Print differences
• Option for forcing (ignore differences)
• Option for silent mode
• Use in fetch_satnogs_db.py
• Convert scripts to .in and include in cmake process, process executable name as cmake variable
• Find better name for transponder utility