CURLINFO_USED_PROXY: return bool whether the proxy was used

Adds test536 to verify

Closes #12719

lib/connect.c

@@ -158,6 +158,13 @@ void Curl_persistconninfo(struct Curl_easy *data, struct connectdata *conn,
   data->info.conn_primary_port = conn->port;
   data->info.conn_remote_port = conn->remote_port;
   data->info.conn_local_port = local_port;
+  data->info.used_proxy =
+#ifdef CURL_DISABLE_PROXY
+    0
+#else
+    conn->bits.proxy
+#endif
+    ;
 }
 
 static const struct Curl_addrinfo *

lib/getinfo.c

@@ -180,7 +180,6 @@ static CURLcode getinfo_char(struct Curl_easy *data, CURLINFO info,
     *param_charp = NULL;
 #endif
     break;
-
   default:
     return CURLE_UNKNOWN_OPTION;
   }
@@ -334,6 +333,15 @@ static CURLcode getinfo_long(struct Curl_easy *data, CURLINFO info,
   case CURLINFO_PROTOCOL:
     *param_longp = data->info.conn_protocol;
     break;
+  case CURLINFO_USED_PROXY:
+    *param_longp =
+#ifdef CURL_DISABLE_PROXY
+      0
+#else
+      data->info.used_proxy
+#endif
+      ;
+    break;
   default:
     return CURLE_UNKNOWN_OPTION;
   }

lib/urldata.h

@@ -1167,6 +1167,7 @@ struct PureInfo {
   CURLproxycode pxcode;
   BIT(timecond);  /* set to TRUE if the time condition didn't match, which
                      thus made the document NOT get fetched */
+  BIT(used_proxy); /* the transfer used a proxy */
 };