Specify work directory

[cypheratheist]

I rebuild packages a lot sometimes and I don't want to wear down my disk more than I need to, so I always specify the work directory to point to a ram drive (also speeds up builds significantly). On my NetBSD systems I have a slightly modified pkg_comp which introduces a REAL_WORKDIR for this purpose.

It would be neat if pkg_comp 2.0 could do this out-of-the-box.