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/http2.c

@@ -193,7 +193,7 @@ static void drain_stream(struct Curl_cfilter *cf,
                          struct Curl_easy *data,
                          struct stream_ctx *stream)
 {
-  int bits;
+  unsigned char bits;
 
   (void)cf;
   bits = CURL_CSELECT_IN;