Handle builds with no SDR support better.

Reorder --ifile pseudo-sdr after the "none" SDR so that a build with no real
SDR support defaults to "none" not --ifile

If the "none" SDR was selected and network mode is not enabled, tell the
user about the problem rather than just failing to do anything useful.

dump1090.c

@@ -603,6 +603,15 @@ int main(int argc, char **argv) {
         }
     }
 
+    if (Modes.sdr_type == SDR_NONE && !Modes.net) {
+        fprintf(stderr,
+                "No SDR available and network mode not enabled; nothing to do!\n"
+                "Select a SDR type (--device-type or --ifile), or enable network mode (--net)\n"
+                "Try %s --help for full option help.\n",
+                argv[0]);
+        exit(1);
+    }
+
 #ifdef _WIN32
     // Try to comply with the Copyright license conditions for binary distribution
     if (!Modes.quiet) {showCopyright();}

sdr.c

@@ -98,8 +98,8 @@ static sdr_handler sdr_handlers[] = {
     { "limesdr", SDR_LIMESDR, limesdrInitConfig, limesdrShowHelp, limesdrHandleOption, limesdrOpen, limesdrRun, limesdrClose },
 #endif
 
-    { "ifile", SDR_IFILE, ifileInitConfig, ifileShowHelp, ifileHandleOption, ifileOpen, ifileRun, ifileClose },
     { "none", SDR_NONE, noInitConfig, noShowHelp, noHandleOption, noOpen, noRun, noClose },
+    { "ifile", SDR_IFILE, ifileInitConfig, ifileShowHelp, ifileHandleOption, ifileOpen, ifileRun, ifileClose },
 
     { NULL, SDR_NONE, NULL, NULL, NULL, NULL, NULL, NULL } /* must come last */
 };