Make the sysctl(8) for checkutf8 positively defined and improve
the description of it.

Submitted by:	Ronald Klop <ronald-lists@klop.ws>
Reviewed by:	rmacklem
Approved by:	rmacklem
Sponsored by:	QNAP Systems Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273202 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f