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/mime.c

@@ -2037,7 +2037,7 @@ static CURLcode cr_mime_read(struct Curl_easy *data,
     if(ctx->total_len >= 0)
       ctx->seen_eos = (ctx->read_len >= ctx->total_len);
     *pnread = nread;
-    *peos = ctx->seen_eos;
+    *peos = (bool)ctx->seen_eos;
     break;
   }