xace: Add an access_mode field to the extension structure.

This allows the same callback to be used for both extension hooks.

Xext/xacestr.h

@@ -97,6 +97,7 @@ typedef struct {
 typedef struct {
     ClientPtr client;
     ExtensionEntry *ext;
+    Mask access_mode;
     int status;
 } XaceExtAccessRec;