MFC r248302:

Update to the latest (un)vis(3) sources from NetBSD.  This adds
multibyte support[0] and the new functions strenvisx and strsenvisx.

Add MLINKS for vis(3) functions add by this and the initial import from
NetBSD[1].

PR:		bin/166364, bin/175418
Submitted by:	"J.R. Oldroyd" <fbsd@opal.com>[0]
		stefanf[1]
Obtained from:	NetBSD
MFC r248303:

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