MINOR: htx: Add a flag on HTX to known when a response was generated by HAProxy

The flag HTX_FL_PROXY_RESP is now set on responses generated by HAProxy,
excluding responses returned by applets and services. It is an informative flag
set by the applicative layer.
diff --git a/include/common/htx.h b/include/common/htx.h
index 5527a47..b22da99 100644
--- a/include/common/htx.h
+++ b/include/common/htx.h
@@ -149,6 +149,7 @@
 #define HTX_FL_PARSING_ERROR     0x00000001 /* Set when a parsing error occurred */
 #define HTX_FL_PROCESSING_ERROR  0x00000002 /* Set when a processing error occurred */
 #define HTX_FL_UPGRADE           0x00000004 /* Set when an upgrade is in progress */
+#define HTX_FL_PROXY_RESP        0x00000008 /* Set when the response was generated by HAProxy */
 
 
 /* HTX block's type (max 15). */