add define for reserved group data type

examples/companion_radio/MyMesh.cpp

@@ -1131,7 +1131,7 @@ void MyMesh::handleCmdFrame(size_t len) {
     ChannelDetails channel;
     if (!getChannel(channel_idx, channel)) {
       writeErrFrame(ERR_CODE_NOT_FOUND); // bad channel_idx
-    } else if (data_type == 0) {
+    } else if (data_type == DATA_TYPE_RESERVED) {
       writeErrFrame(ERR_CODE_ILLEGAL_ARG);
     } else if (payload_len > MAX_CHANNEL_DATA_LENGTH) {
       MESH_DEBUG_PRINTLN("CMD_SEND_CHANNEL_DATA payload too long: %d > %d", payload_len, MAX_CHANNEL_DATA_LENGTH);

src/helpers/TxtDataHelpers.h

@@ -6,6 +6,7 @@
 #define TXT_TYPE_PLAIN          0      // a plain text message
 #define TXT_TYPE_CLI_DATA       1      // a CLI command
 #define TXT_TYPE_SIGNED_PLAIN   2      // plain text, signed by sender
+#define DATA_TYPE_RESERVED      0x0000 // reserved for future use
 #define DATA_TYPE_DEV           0xFFFF // developer namespace for experimenting with group/channel datagrams and building apps
 
 class StrHelper {