MINOR: lua: add utility function for check boolean argument

Strangely, the Lua API doesn't provides a function like
luaL_checkboolean(). This little function add this one.

include/proto/hlua_fcn.h

@@ -1,6 +1,8 @@
 #ifndef _PROTO_HLUA_FCN_H
 #define _PROTO_HLUA_FCN_H
 
+int hlua_checkboolean(lua_State *L, int index);
+
 void hlua_class_const_int(lua_State *L, const char *name, int value);
 void hlua_class_const_str(lua_State *L, const char *name, const char *value);
 void hlua_class_function(lua_State *L, const char *name, int (*function)(lua_State *L));

src/hlua_fcn.c

@@ -41,6 +41,13 @@ static int class_listener_ref;
 
 static struct field stats[STATS_LEN];
 
+int hlua_checkboolean(lua_State *L, int index)
+{
+	if (!lua_isboolean(L, index))
+		luaL_argerror(L, index, "boolean expected");
+	return lua_toboolean(L, index);
+}
+
 /* This function gets a struct field and convert it in Lua
  * variable. The variable is pushed at the top of the stak.
  */