systemd: Add support for path units.

This allows to define systemd.path(5) units, for example like this:

{
  systemd = let
    description = "Set Key Permissions for xyz.key";
  in {
    paths.set-key-perms = {
      inherit description;
      before = [ "network.target" ];
      wantedBy = [ "multi-user.target" ];
      pathConfig.PathChanged = "/run/keys/xyz.key";
    };

    services.set-key-perms = {
      inherit description;
      serviceConfig.Type = "oneshot";
      script = "chown myspecialkeyuser /run/keys/xyz.key";
    };
  };
}

The example here is actually useful in order to set permissions for the
NixOps keys target to ensure those permisisons aren't reset whenever the
key file is reuploaded.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
haskellPackages.thDesugar: Update to 1.3.1
add v4l2loopback: a kernel module to create V4L2 loopback devices
haskell-mtl: update to version 2.1.3.1
haskell-streaming-commons: add version 0.1.0.2
haskell-conduit-extra: add version 1.0.0
haskell-stm: add version 2.4.3
haskell-case-insensitive: add version 1.2.0.0
haskell-haddock: add version 2.14.1
haskell-HList: update to version 0.3.4.1
haskell-authenticate: update to version 1.3.2.8
haskell-basic-prelude: update to version 0.3.7
haskell-classy-prelude: update to version 0.8.2
haskell-cryptohash-conduit: update to version 0.1.1
haskell-html-conduit: update to version 1.1.0.4
haskell-http-reverse-proxy: update to version 0.3.1.4
haskell-language-c: update to version 0.4.3
haskell-monad-logger: update to version 0.3.4.1
haskell-mono-traversable: update to version 0.4.0.4
haskell-persistent-postgresql: update to version 1.3.0.5