Improve Gentoo support

This PR improves some Gentoo related items:

  * Don't hardcode all dependencies in the bootstrapping script, but instead let Gentoo's package manager (Portage) handle this on its own by writing to the corresponding /etc/portage/package.* files
  * Use `eix` where available
  * Use the package category when installing 'salt' to prevent possible naming collisions
  * Add support for systemd on Gentoo (the Gentoo ebuild still needs to catch up and install the systemd units)
  * Don't let the bootstrapping script pollute the worldfile with salt dependencies