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/imap.c

@@ -710,7 +710,8 @@ static CURLcode imap_perform_authentication(struct Curl_easy *data,
   }
 
   /* Calculate the SASL login details */
-  result = Curl_sasl_start(&imapc->sasl, data, imapc->ir_supported, &progress);
+  result = Curl_sasl_start(&imapc->sasl, data, (bool)imapc->ir_supported,
+                           &progress);
 
   if(!result) {
     if(progress == SASL_INPROGRESS)