unvis(1) on OpenBSD

The text a71e9dd252d9, as OpenBSD 4.1, OpenBSD 4.0, OpenBSD 3.9, OpenBSD 3.8, OpenBSD 3.7, OpenBSD 3.6, OpenBSD 3.5, OpenBSD 3.4 shipped it.

revert a visual representation of data back to original form