systemd: Allow customisation of upstream units

You can now say:

  systemd.services.foo.baseUnit = "${pkgs.foo}/.../foo.service";

This will cause NixOS' generated foo.service file to include
foo.service from the foo package.  You can then apply local
customization in the usual way:

  systemd.services.foo.serviceConfig.MemoryLimit = "512M";

Note however that overriding options in the original unit may not
work.  For instance, you cannot override ExecStart.

It's also possible to customize instances of template units:

  systemd.services."getty@tty4" =
    { baseUnit = "/etc/systemd/system/getty@.service";
      serviceConfig.MemoryLimit = "512M";
    };

This replaces the unit options linkTarget (which didn't allow
customisation) and extraConfig (which did allow customisation, but in
a non-standard way).