Use short distro name, if the linux distro has no codename
Add support for pacman
Only use apt when apt-get is available.

Avoid assuming all Linux machines are Ubuntu/Debian flavours.
First cut at installs_with_pacman predicates.
Merge branch 'fhahn-arch-linux'

* fhahn-arch-linux:
  First cut at installs_with_pacman predicates.
  Only use apt when apt-get is available.
  Add support for pacman
  Use short distro name, if the linux distro has no codename