lua: restore return value of require('uci') to module table.

Before commit "b121dd lua: add lua 5.2 compatibility changes", the
module table was filled by luaL_register() which would leave the table
at the top of the stack.  But in current code the module table will be
popped by lua_setglobal() causing the return value defaulting to boolean
value true.

Also fix the return value of luaopen_uci() to 1 to indicate the number
of return value on stack.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

lua/uci.c

@@ -994,8 +994,9 @@ luaopen_uci(lua_State *L)
 
 	/* create module */
 	lua_newtable(L);
+	lua_pushvalue(L, -1);
 	luaL_setfuncs(L, uci, 0);
 	lua_setglobal(L, MODNAME);
 
-	return 0;
+	return 1;
 }