Bug #261377 - Add a new 'egencache' tool to generate metadata cache for

Bug #261377 - Add a new 'egencache' tool to generate metadata cache for
distribution. It only have the most basic functionality now, and more
features will be added later.

Usage: egencache [options] --update [atom] ...

Options:
  -h, --help            show this help message and exit
  --update              update metadata/cache/ (generate as necessary)
  --cache-dir=CACHE_DIR
                        location of the metadata cache
  --config-root=CONFIG_ROOT
                        location of portage config files
  --jobs=JOBS           max ebuild processes to spawn
  --load-average=LOAD_AVERAGE
                        max load allowed when spawning multiple jobs
(trunk r13260)

svn path=/main/branches/2.1.6/; revision=13439