install: Make the setup_symlink function more flexible.
install: Setup eatmydata for apt-get, aptitude, dpkg.
Merge branch 'speedup_dpkg_apt'

This branch sets up eatmydata for apt-get, aptitude, and dpkg. This is
especially useful on Debian-based systems where I'm using btrfs for /. It
appears that btrfs has horrible performance with sync() calls which dpkg
uses quite liberally.