OpenBSD pkg_tools support

This adds support for `pkg_add`, `pkg_delete`, `pkg_info`, etc on OpenBSD, collectively known as `pkg_tools`.

In addition, I solved a bug found while trying to use it on OpenBSD, which is the shebang used on the scripts. Using `#!/bin/bash` on OpenBSD (and confirmed on FreeBSD, actually) doesn't work, because it's installed to /usr/local/bin/bash; so, `#!/usr/bin/env bash` is used instead, which should be compatible with just about any common *nix system.

I've tested it on my OpenBSD server, so it should be fine.