Support f00b4r0 acarsdec fork and fix ADS-B stop

ACARS (f00b4r0/DragonOS compatibility):
- Use --output json:file (not json:file:-) for stdout
- Use --rtlsdr instead of -r for device selection
- Use -m 256 for 3.2 MS/s sample rate (wider bandwidth for NA freqs)
- Properly detects fork by checking for --output in help

The f00b4r0 fork (used by DragonOS) has different CLI syntax than
TLeconte's original. Key differences:
  - TLeconte: -j -r <device>
  - f00b4r0:  --output json:file -m 256 --rtlsdr <device>

ADS-B stop fix:
- Add Content-Type header to stop fetch request
- Flask's request.json requires application/json content type
- Without this header, stop returns HTTP 415 and dump1090 keeps running

templates/adsb_dashboard.html

@@ -2466,7 +2466,11 @@ sudo make install</code>
                     const url = useAgent
                         ? `/controller/agents/${adsbCurrentAgent}/adsb/stop`
                         : '/adsb/stop';
-                    await fetch(url, { method: 'POST' });
+                    await fetch(url, {
+                        method: 'POST',
+                        headers: { 'Content-Type': 'application/json' },
+                        body: JSON.stringify({})
+                    });
 
                     // Update agent running modes tracking
                     if (useAgent && typeof agentRunningModes !== 'undefined') {