turn on -Werror and fix resulting errors

src/modules/mavlink/mavlink_ftp.cpp

@@ -801,7 +801,7 @@ MavlinkFTP::_return_request(Request *req)
 
 /// @brief Copy file (with limited space)
 int
-MavlinkFTP::_copy_file(const char *src_path, const char *dst_path, ssize_t length)
+MavlinkFTP::_copy_file(const char *src_path, const char *dst_path, size_t length)
 {
 	char buff[512];
 	int src_fd = -1, dst_fd = -1;

src/modules/mavlink/mavlink_ftp.h

@@ -142,7 +142,7 @@ private:
 	static void	_worker_trampoline(void *arg);
 	void		_process_request(Request *req);
 	void		_reply(Request *req);
-	int		_copy_file(const char *src_path, const char *dst_path, ssize_t length);
+	int		_copy_file(const char *src_path, const char *dst_path, size_t length);
 
 	ErrorCode	_workList(PayloadHeader *payload);
 	ErrorCode	_workOpen(PayloadHeader *payload, int oflag);

src/modules/mavlink/mavlink_main.cpp

@@ -1638,7 +1638,7 @@ Mavlink::start(int argc, char *argv[])
 		       SCHED_PRIORITY_DEFAULT,
 		       2800,
 		       (main_t)&Mavlink::start_helper,
-		       (const char **)argv);
+		       (char * const *)argv);
 
 	// Ensure that this shell command
 	// does not return before the instance

src/modules/mavlink/mavlink_messages.cpp

@@ -1764,6 +1764,9 @@ protected:
 				case RC_INPUT_SOURCE_PX4IO_ST24:
 					msg.rssi |= (3 << 4);
 					break;
+				case RC_INPUT_SOURCE_UNKNOWN:
+					// do nothing
+					break;
 			}
 
 			if (rc.rc_lost) {