Don't set auto_name_dirs because it messes up prompts

From http://zsh.sourceforge.net/Doc/Release/Options.html#Completion-4

> AUTO_NAME_DIRS
> Any parameter that is set to the absolute name of a directory immediately
> becomes a name for that directory, that will be used by the ‘%~’ and
> related prompt sequences, and will be available when completion is performed
> on a word starting with ‘~’.
> (Otherwise, the parameter must be used in the form ‘~param’ first.)

Explained in more detail in
https://github.com/wayneeseguin/rvm/issues/3091#issuecomment-60083194

Related issues:
https://github.com/robbyrussell/oh-my-zsh/issues/2857
https://github.com/robbyrussell/oh-my-zsh/issues/3238
https://github.com/wayneeseguin/rvm/issues/3091
Merge pull request #3252 from mcornella/fix-auto-named-dirs-prompt

Don't set auto_name_dirs because it messes up prompts
Respect original PATH definitions on install

The exporting of PATH as a fixed string would clobber any system-wide
paths added to env after installation. Instead, we now read any existing
PATH exports from previous config files and copy those from the user's
bash config, or else default to a basic prepend-only example that
respects the system environment.