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/urldata.h

@@ -724,8 +724,8 @@ struct connectdata {
   /* HTTP version last responded with by the server or negotiated via ALPN.
    * 0 at start, then one of 09, 10, 11, etc. */
   uint8_t httpversion_seen;
-  uint8_t connect_only;
   uint8_t gssapi_delegation; /* inherited from set.gssapi_delegation */
+  BIT(connect_only);
 };
 
 #ifndef CURL_DISABLE_PROXY