Support a new 'l'/'L' Beast option; when enabled, send only locally-generated messages

[mutability]

As an implementation detail, whenever 'L' is enabled, 'V' (verbatim) is also implicitly enabled. (Clients should not rely on this as it will probably change in the future -- they should request both local and verbatim mode if they do want both)

See #202 for background.

[mutability]

@nunojpg does this work as a first step for what you need?

(caution: compiles, but otherwise untested!)

[nunojpg]

Would the following work for a info message? I would submit a PR on top of this.

--- a/net_io.c
+++ b/net_io.c
@@ -339,6 +339,26 @@ static void modesCloseClient(struct client *c) {
 
     autoset_modeac();
 }
+
+static void modesWriteStatusClient(struct client *c) {
+    char str[] =
+        "\x1a\xff"
+        MODES_DUMP1090_VARIANT
+        ","
+        MODES_DUMP1090_VERSION
+        ","
+        "0";
+    str[sizeof(str)-2] = c->local_requested ? 'l' : (c->verbatim_requested ? 'v' : 'n');
+
+#ifndef _WIN32
+    int nwritten = write(c->fd, str, sizeof(str) -1 );
+#else
+    int nwritten = send(c->fd, str, sizeof(str) - 1, 0 );
+#endif
+    if (nwritten != sizeof(str) - 1) {
+        modesCloseClient(c);
+    }
+}
 //
 //=========================================================================
 //
@@ -1220,6 +1240,9 @@ static int handleBeastCommand(struct client *c, char *p) {
         c->local_requested = 1;
         handleOptionsChange(c);
         break;
+    case 'i':
+        modesWriteStatusClient(c);
+        break;
     }
 
     return 0;

[mutability]

Would the following work for a info message?

Yes that's the rough approach I was thinking of.

I'll merge this PR now so I can get the release out the door and we can work out the details of the info message in a separate PR?