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. 
Sorry, maybe I gave bad advise earlier.

Amusingly, I think this syntax (which indeed is already used elsewhere in this script) appears not to work for me. It appears that the `True/False` values are transposed. By that I mean, this function needs to return `0` on `True` rather than `1` like it does. Perhaps I'm mistaken. Unfortunately I can't look any deeper at the moment.

@Jchase2 can you test this and check whether it works as intended? Maybe I'm just mistaken. Both testing this PR and googling bash syntax are quite inconvenient for me at present. Some bash guru reading this is chuckling to himself just now but at the moment I don't know whether this function needs to return `0` or `1` for a `Yes` input. I think it needs to return `0` but our existing script says `1` so I hesitate...