Rework finding a writable cache directory

This is a refactor and refresh of the code used to find where we should
download packages.

* Incorporate suggestions from FS#25435 to use TMPDIR from the
  environment if set, otherwise fall back to /tmp as before.
* Make the writability tests a bit more in depth. We now do a three part
  check consisting of:
  - S_ISDIR(): is this even a directory
  - access(W_OK): is this directory writable by the current user.
    Unfortunately for root, this almost always returns that it is, but
    in the case of a RO mount or NFS share inaccessible to root, this
    check will exclude the directory.
  - mode & (any write bit): is there a writable bit set on this
    directory. This makes it possible to enforce a read-only cache
    directory by setting permissions to 0555, for example.

Signed-off-by: Dan McGee <dan@archlinux.org>

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