Don't mount /sys/fs/fuse/connections and /sys/kernel/config

These fail to mount if you don't have the appropriate kernel support,
and this confuses NixOps' ‘check’ command. We should teach NixOps not
to complain about non-essential mount points, but in the meantime it's
better to turn them off.
switch-to-configuration: Use old systemctl to stop units

Otherwise, when switching from systemd 203 to 212, you get errors like:

  Failed to stop remote-fs.target: Bad message
  Failed to stop systemd-udevd-control.socket: Bad message
  ...
nixos: Add support for changing supported systems

release.nix and release-combined.nix current hardcode the systems which
they are built for. This change introduces an argument to the
expressions called supportedSystems, which allows the builder to choose
which architectures he wants to build. By default, this uses the same
linux x86_64 and i686 architectures.