RC_Channel: added RC_OPTION bit for allowing RC protocol switching

this is meant to prevent accidential switching to a disconnected pin

libraries/RC_Channel/RC_Channel.h

@@ -477,7 +477,9 @@ public:
         return get_singleton() != nullptr && (_options & uint32_t(Option::SUPPRESS_CRSF_MESSAGE));
     }
 
-
+    bool allow_rc_protocol_switching() const {
+        return _options & uint32_t(Option::ALLOW_RC_SWITCHING);
+    }
 
     // returns true if overrides should time out.  If true is returned
     // then returned_timeout_ms will contain the timeout in
@@ -537,6 +539,7 @@ protected:
         ALLOW_SWITCH_REV        = (1U << 7), // honor the reversed flag on switches
         CRSF_CUSTOM_TELEMETRY   = (1U << 8), // use passthrough data for crsf telemetry
         SUPPRESS_CRSF_MESSAGE   = (1U << 9), // suppress CRSF mode/rate message for ELRS systems
+        ALLOW_RC_SWITCHING      = (1U << 10), // allow switching RC protocols
     };
 
     void new_override_received() {

libraries/RC_Channel/RC_Channels_VarInfo.h

@@ -89,7 +89,7 @@ const AP_Param::GroupInfo RC_Channels::var_info[] = {
     // @DisplayName: RC options
     // @Description: RC input options
     // @User: Advanced
-    // @Bitmask: 0:Ignore RC Receiver, 1:Ignore MAVLink Overrides, 2:Ignore Receiver Failsafe bit but allow other RC failsafes if setup, 3:FPort Pad, 4:Log RC input bytes, 5:Arming check throttle for 0 input, 6:Skip the arming check for neutral Roll/Pitch/Yay sticks, 7:Allow Switch reverse, 8:Use passthrough for CRSF telemetry, 9:Suppress CRSF mode/rate message for ELRS systems
+    // @Bitmask: 0:Ignore RC Receiver, 1:Ignore MAVLink Overrides, 2:Ignore Receiver Failsafe bit but allow other RC failsafes if setup, 3:FPort Pad, 4:Log RC input bytes, 5:Arming check throttle for 0 input, 6:Skip the arming check for neutral Roll/Pitch/Yay sticks, 7:Allow Switch reverse, 8:Use passthrough for CRSF telemetry, 9:Suppress CRSF mode/rate message for ELRS systems, 10:Enable RC Protocol re-detection
     AP_GROUPINFO("_OPTIONS", 33, RC_CHANNELS_SUBCLASS, _options, (uint32_t)RC_Channels::Option::ARMING_CHECK_THROTTLE),
 
     // @Param: _PROTOCOLS