[arch-projects] [devtools] Minor cleanups of common.sh

I'm the maintainer of the equivalent of devtools in Parabola, and I realized that most of the changes we apply have never been pushed upstream. So I'm doing that now. This is a set of minor improvements to common.sh that I hope are fairly uncontroversial; even if they aren't directly taken advantage of right now in devtools. In the coming days, I will be sending more patches as I isolate out many years of `git merge`s into sets of atomic commits that I think you might welcome. -- Happy hacking, ~ Luke Shumaker