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/ratelimit.c

@@ -189,7 +189,7 @@ bool Curl_rlimit_active(struct Curl_rlimit *r)
 
 bool Curl_rlimit_is_blocked(struct Curl_rlimit *r)
 {
-  return r->blocked;
+  return (bool)r->blocked;
 }
 
 int64_t Curl_rlimit_avail(struct Curl_rlimit *r,