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. 
Actually I'm just now noticing that the existing `confirm` function defaults to True if you enter a nonfalse input. That's sort of not cool so feel free to use @gubatron's syntax to modify the `confirm` function and then make use of the `confirm` function here.

Also feel free to leave that for a separate PR and just go ahead and use the `confirm` function as-is for now. Either way.

In any case, the current syntax on this line in this PR doesn't work for me.