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).
Initialize installed with a space, and always append a space after the package name when adding it, then you can do this:
```
if [ "${installed#*" $PACKAGE "}" != "$installed" ]; then
    continue
fi
```