Move SpooledTempFile to /var/tmp by default on *nix

[Stebalien]

At the moment, the backing file file is created in the default temporary directory. However, on many linux systems, this is ram-backed by default.

We should either:

1. Special-case SpooledTempFile and store files in /var/tmp by default on all unix platforms (other than macos?).
2. Provide an additional method to create the spooled temporary file in the right "on-disk" temporary directory in a platform independent way.

[Stebalien]

Unfortunately, there's no reasonable way to do this without also ignoring, e.g., the TMPDIR environment variable. I guess I could use /var/tmp if TMPDIR is unset but... meh.