Archlinux support

This adds support for Arch Linux/Arch Linux ARM. It has been around in my repo for a long time now, with a good number of users I believe, and rarely has issues.

First 3 commits are not directly related to arch, but necessary (especially the last 2).
You can use the same language as mount-chroot:
`Initializing pacman keyring (move the mouse around to generate entropy)...`