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/http.c

@@ -527,7 +527,7 @@ static bool http_should_fail(struct Curl_easy *data, int httpcode)
     return TRUE;
 #endif
 
-  return data->state.authproblem;
+  return (bool)data->state.authproblem;
 }
 
 /*
@@ -806,7 +806,7 @@ Curl_http_output_auth(struct Curl_easy *data,
 #ifndef CURL_DISABLE_PROXY
   /* Send proxy authentication header if needed */
   if(conn->bits.httpproxy &&
-     (conn->bits.tunnel_proxy == (bit)proxytunnel)) {
+     (conn->bits.tunnel_proxy == (curl_bit)proxytunnel)) {
     result = output_auth_headers(data, conn, authproxy, request, path, TRUE);
     if(result)
       return result;