[arch-general] Check for update in script

[Yamakaky]

> It can certainly be done. You can always do a "pacman -Su" later, if
> you want.

The problem is that I want to check for updates in a script. 'pacman
-Sy' without upgrade can cause problems.

> In your use case, you can probably specify an alternative db path
> using the --dbpath switch in pacman.

That's what I wanted to do, but dbpath countains the local and upstream 
databases, so I thought it might cause problems.