server: replace --readonly with --writable

src/server.h

@@ -74,7 +74,7 @@ struct server {
   int sig_code;            // close signal
   char sig_name[20];       // human readable signal string
   bool url_arg;            // allow client to send cli arguments in URL
-  bool readonly;           // whether not allow clients to write to the TTY
+  bool writable;           // whether clients to write to the TTY
   bool check_origin;       // whether allow websocket connection from different origin
   int max_clients;         // maximum clients to support
   bool once;               // whether accept only one client and exit on disconnection