lib: add CURLINFO_CONN_ID and CURLINFO_XFER_ID

- add an `id` long to Curl_easy, -1 on init
- once added to a multi (or its own multi), it gets
  a non-negative number assigned by the connection cache
- `id` is unique among all transfers using the same
  cache until reaching LONG_MAX where it will wrap
  around. So, not unique eternally.
- CURLINFO_CONN_ID returns the connection id attached to
  data or, if none present, data->state.lastconnect_id
- variables and type declared in tool for write out

Closes #11185

lib/imap.c

@@ -1777,7 +1777,7 @@ static CURLcode imap_sendf(struct Curl_easy *data, const char *fmt, ...)
 
   /* Calculate the tag based on the connection ID and command ID */
   msnprintf(imapc->resptag, sizeof(imapc->resptag), "%c%03d",
-            'A' + curlx_sltosi(data->conn->connection_id % 26),
+            'A' + curlx_sltosi((long)(data->conn->connection_id % 26)),
             ++imapc->cmdid);
 
   /* start with a blank buffer */