Asking user if we should upgrade on arch install.

pacman -Sy can cause dependency problems and conflicts, as noted in #944 

So, this replaces pacman -Sy with -Syu (system upgrade), but prompts the user as to whether they want to upgrade and warns that if they haven't updated their package list recently, downloads/installing may fail. 
That look ok? I think everyone else was using 2 spaces so I just changed it to fit in with that.