Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Replace our (un)vis(1) commands with implementations from NetBSD to
match our import of the (un)vis(3) APIs.

This adds support for multibyte encoding and the -h and -m flags which
support HTTP and MIME encoding respectively.

PR: bin/175418
Obtained from: NetBSD

Vendor import of NetBSD's unvis(1) at 2013-02-21