Add formula for netcat6

Formula for netcat6, a fork of netcat with better IPv6 support than GNU netcat (which is already in homebrew), as well as some additional features (like file-transfer mode and half-close support).

Upstream's tarball is broken at the moment (for the first time I can remember in the last 7 years or so), so I put in the debian pool as a mirror and e-mailed upstream to get the tarballs back.