Issue with permissions and case-insensitive globbing

[Devin Hussey <husseydevin@xxxxxxxxx>]

Case insensitive globbing will not work at all if any of the parent
directories of the target folder are inaccessible.

This bug makes zsh very unstable on Termux (a Linux environment for
Android), as scripts which use "~/..." will immediately show no
results.

Android's directory structure makes it so apps can only access their
/data/data subfolder, blocking access to /data/data itself (it will
return EACCES).

Termux's $HOME is /data/data/com.termux/files/home.

The simplified folder setup is this:

root    root    0700 /
root    root    0700 /data
root    root    0700 /data/data
termux termux 0700 /data/data/com.termux

A (possible) fix for this issue would be to start in the "base"
directory before any opendir() calls instead of starting from the root
directory, and checking for errno.

See:

 - https://github.com/sorin-ionescu/prezto/issues/1560
- https://github.com/termux/termux-packages/issues/1894