MINOR: lua: adds "forced yield" flag

This flag indicate that the current yield is returned by the Lua
execution task control. If this flag is set, the current task may
quit but will be set in the run queue to be re-executed immediatly.

This patch modify the "hlua_yieldk()" function, it adds an argument
that contain a field containing yield options.
diff --git a/include/types/hlua.h b/include/types/hlua.h
index 0e1b2fc..58402f4 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -17,6 +17,7 @@
 struct session;
 
 #define HLUA_RUN       0x00000001
+#define HLUA_CTRLYIELD 0x00000002
 
 enum hlua_exec {
 	HLUA_E_OK = 0,