Gentoo fixes

Gentoo fixes

- Use boot use flag for systemd instead of gnuefi
- Add --deep to emerge invocation to fix dep resolution failures
- Use curl to download stage3 tarball so we get a progress bar
- Do not exclude dev, proc and sys directories when extracting tarball
  (only exclude their contents)
- Copy pkgmngr/ directory into stage3/ directory wholesale instead of
  individual files
- Various coding style fixes
- Stop using Repositories= to specify binary package repositories as it
  is not its intended purpose. Instead, pass configured environment
  variables to emerge so users can set PORTAGE_BINHOST instead.