Warn if doing JSON output and either -d or -v are specified.
Debugging and/or verbose output can get in the way of producing
valid/parsable JSON.

Inspired by #737.
diff --git a/src/iperf_api.c b/src/iperf_api.c
index 58d4479..8b65d23 100755
--- a/src/iperf_api.c
+++ b/src/iperf_api.c
@@ -1231,6 +1231,14 @@
         warning("Report format (-f) flag ignored with JSON output (-J)");
     }
 
+    /* Show warning if JSON output is used with verbose or debug flags */
+    if (test->json_output && test->verbose) {
+        warning("Verbose output (-v) may interfere with JSON output (-J)");
+    }
+    if (test->json_output && test->debug) {
+        warning("Debug output (-d) may interfere with JSON output (-J)");
+    }
+
     return 0;
 }