At the moment, this code is looking at the stat output to determine if a
temp directory can be written to. However, just because the stat bits
say that a directory is writable, does not make it so; and, likewise,
the operating system may in fact grant access to paths that the stat
bits and process UID say should be inaccessible.

These systems include:

* Posix ACL's
* Linux's capabilities like CAP_DAC_OVERRIDE
* Linux Security Modules like SELinux or AppArmor
* Syscall filters like Linux's seccomp
* Granular capability systems like FreeBSD's Capsicum
* OpenBSD's pledge and unveil
* Windows too has a rich ACL system for controlling filesystem access

The best thing to do is simply to try and access the path with
`File.writable?` and let the operating system tell us if the path can be
accessed.