Add primus

Adds primus: https://github.com/amonakov/primus

For a 64bit + 32bit system the LD_LIBRARY_PATH must contain both the 32bit and 64bit primus libraries. Providing a different primusrun for each architecture does not work as expected. Using steam under wine, for instance, can involve both 32bit and 64bit process. All of which inherit the same LD_LIBRARY_PATH.

Perhaps this should only be done if a config is set?

Both bumblebee and virtualgl will need similar setups. Full support requires at least bumblebee to be changed. Even so, I'd like to get this pull request accepted before completing bumbleebee and virtualgl. That way I can incorporate any changes into those developments.