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_proxy.c

@@ -187,7 +187,7 @@ void Curl_http_proxy_get_destination(struct Curl_cfilter *cf,
   if(*phostname != cf->conn->host.name)
     *pipv6_ip = (strchr(*phostname, ':') != NULL);
   else
-    *pipv6_ip = cf->conn->bits.ipv6_ip;
+    *pipv6_ip = (bool)cf->conn->bits.ipv6_ip;
 }
 
 struct cf_proxy_ctx {