bumblebee-git: modernize packaging

Synced with community/bumblebee 3.2.1-8. Remaining changes:

 - trailing whitespace removed
 - keep nouveau optdepends, it could still work.
 - change version number to include 'r' to avoid treating
   '3.2-2-g123abcd' as newer version than '3.2.1'.
 - removed nvidia-bumblebee-utils dependency, you can now use the
   official nvidia-utils package.
 - kept 99-bumblebee-nvidia-dev.rules which existed before.

Merged changes:

 - pulled more nvidia-utils-XXX deps
 - added help2man dependency for manpage.
 - install to /usr/bin.
 - use nvidia-utils instead of nvidia-utils-bumblebee (adjusted paths
   accordingly).
bumblebee-git: make virtualgl optional

primus works as good, do not hard-depend on virtualgl.