special case permissions when creating /opt

[rolandwalker]

or any other toplevel directory. Closes #4187

[twhittock]

This fixes the default case, but you should be aware that if the caskroom is anything deeper than /toplevel/caskroom eg /opt/foo/bar/caskroom - then the intermediate directories would also have the root owner and permission bits which would render the caskroom inaccessible.

I'd recommend using mkdir -m 0755 #{caskroom} in the sudo command followed by the appropriate chown and possibly a chmod using the current umask for the parent dir.

[rolandwalker]

Thanks for noting these specific points for the future, @twhittock . I am intentionally making the patch absolute minimum impact, per the reasons in #4187. Oh, and thanks again for the original report!