gnu: Add GNUnet.

* gnunet/packages/gnunet.scm (gnunet): New variable.
* gnu/package/patches/gnunet-fix-scheduler.patch: New file.
* gnu/package/patches/gnunet-fix-tests.patch: New file.
* gnu-system.am (dist_patch_DATA): Add the above two patch files.

Co-authored-by: Andreas Enge <andreas@enge.fr>
Signed-off-by: Ludovic Courtès <ludo@gnu.org>