Allow invocation of subcommand with full paths

Full paths to subcommand source and Ruby may be given,
which is helpful in troubleshooting, particularly for
forcing a specific Ruby version.

Example usage:
```bash
$ HOMEBREW_BREW_FILE=/usr/local/bin/brew /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby /usr/local/Library/brew.rb /usr/local/bin/brew-cask.rb help
```

Previously, this would work, but throw an odd error when
returning to Homebrew.  Fix: we unconditionally do not
return to Homebrew.