nvidia-x11: change meta.license from "unfree" to "proprietary"

I'm not entirely sure what the appropriate license attribute for this
package is. The license [1] says:

 | 2.1.2 Linux/FreeBSD Exception. Notwithstanding the foregoing terms of
 | Section 2.1.1, SOFTWARE designed exclusively for use on the Linux or
 | FreeBSD operating systems, or other operating systems derived from
 | the source code to these operating systems, may be copied and
 | redistributed, provided that the binary files thereof are not
 | modified in any way (except for unzipping of compressed files).

It sounds to me like this gives NixOS the right to re-distribute the
files (because we don't modify them). The 'proprietary' license sort-of
fits that. On the other hand, we seem to assume that proprietary
software cannot be redistributed, which doesn't apply here.

[1] http://www.nvidia.com/content/DriverDownload-March2009/licence.php?lang=us