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. 
`||` doesn't work on my system but `-o` does and I think is more bashish.

However there's already a `confirm` function which we should reuse here.

`if confirm ; then`