msvc: drop exception, make BIT() a bitfield with Visual Studio

Add casts to `bool`, or use `bit` type in local variables, where
neccessary to avoid MSVC compiler warnings C4242.

Note: There may remain places needing the above updates, where not
tested in CI, and missed in manual review.

Also:
- urldata: convert struct field `connect_only` to bitfield to match its
  counterpart in another struct.
- rename curl-specific `bit` type to `curl_bit`.

Closes #20142

lib/http2.c

@@ -986,7 +986,8 @@ static CURLcode on_stream_frame(struct Curl_cfilter *cf,
     else
       stream->status_code = -1;
 
-    h2_xfer_write_resp_hd(cf, data, stream, STRCONST("\r\n"), stream->closed);
+    h2_xfer_write_resp_hd(cf, data, stream, STRCONST("\r\n"),
+                          (bool)stream->closed);
 
     if(stream->status_code / 100 != 1) {
       stream->resp_hds_complete = TRUE;