add support for DragonFly and OpenBSD

DragonFly is a FreeBSD derivative, so the same `du` flags as for FreeBSD
and Darwin apply. OpenBSD does not support exclusion.

The correct invocation for in-place `sed` without backup varies. GNU Sed
allows just leaving out the suffix, but BSD Sed requires an empty suffix
(which in turn GNU Sed doesn't accept). OpenBSD Sed has no in-place
option, use `ed` instead.

`mail` works the same on all BSD.

explicitly list `bash` dependency, it's not installed on most BSD by default.