select: switch to macros in uppercase

Curl_select_ready() was the former API that was replaced with
Curl_select_check() a while back and the former arg setup was provided
with a define (in order to leave existing code unmodified).

Now we instead offer SOCKET_READABLE and SOCKET_WRITABLE for the most
common shortcuts where only one socket is checked. They're also more
visibly macros.

lib/url.c

@@ -2892,7 +2892,7 @@ static bool SocketIsDead(curl_socket_t sock)
   int sval;
   bool ret_val = TRUE;
 
-  sval = Curl_socket_ready(sock, CURL_SOCKET_BAD, 0);
+  sval = SOCKET_READABLE(sock, 0);
   if(sval == 0)
     /* timeout */
     ret_val = FALSE;