New function 'luaL_makeseed'

This function unifies code from 'lua_newstate', 'math.randomseed',
and 'table.sort' that tries to create a value with a minimum level
of randomness.

lauxlib.h

@@ -100,6 +100,8 @@ LUALIB_API int (luaL_loadstring) (lua_State *L, const char *s);
 
 LUALIB_API lua_State *(luaL_newstate) (void);
 
+LUALIB_API unsigned int luaL_makeseed (lua_State *L);
+
 LUALIB_API lua_Integer (luaL_len) (lua_State *L, int idx);
 
 LUALIB_API void (luaL_addgsub) (luaL_Buffer *b, const char *s,