Merge branch 'master' into config

Merge branch 'master' into config

* master:
  cmd/{split,save}: support any compression level using the new -# feature.
  options.py: add support for '-#' style compression options.
  Add documentation for compression levels
  Add test case for compression level
  Add compression level options to bup save and bup split
  Make zlib compression level a parameter for Client
  Make zlib compression level a parameter of git.PackWriter
  Use is_superuser() rather than checking euid directly
  Add is_superuser() helper function
  Makefile: add a PREFIX variable for locations other than /usr.