MINOR: lua: add core.done() function

This function immediately give back the control to HAProxy core.
diff --git a/include/types/hlua.h b/include/types/hlua.h
index f481bdf..453980b 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -24,6 +24,7 @@
 #define HLUA_CTRLYIELD 0x00000002
 #define HLUA_WAKERESWR 0x00000004
 #define HLUA_WAKEREQWR 0x00000008
+#define HLUA_EXIT      0x00000010
 
 enum hlua_exec {
 	HLUA_E_OK = 0,