urldata: shrink *select_bits int => unsigned char

- dselect_bits
- cselect_bits

... are using less than 8 bits. Changed types and moved them towards
the end of the structs to fit better.

Closes #11025

lib/multi.c

@@ -3222,7 +3222,7 @@ static CURLMcode multi_socket(struct Curl_multi *multi,
 
         if(data->conn && !(data->conn->handler->flags & PROTOPT_DIRLOCK))
           /* set socket event bitmask if they're not locked */
-          data->conn->cselect_bits = ev_bitmask;
+          data->conn->cselect_bits = (unsigned char)ev_bitmask;
 
         Curl_expire(data, 0, EXPIRE_RUN_NOW);
       }