Merge commit 'acb089b983171667467adc66f56a723b609ed22e' into kbsd/vis

Highlights:
- Some style fixes
- Bumped mbbuf in istrsenvisx() to MB_LEN_MAX to avoid VLAs
- mbslength cannot go negative, so make it unsigned
- Further bounds checking & fix an additional overrun, with dlen == 0
- Avoid duplicate call to wcslen(start)

Update vis(3) the latest from NetBSD.

This adds VIS_DQ for compatiblity with OpenBSD.

Correct by an off-by-one error and a read buffer overflow detected using
asan.

MFC after: 1 day

Update to a June 8th snapshot of (un)vis form NetBSD.

This adds stravis() and some new encoding flags VIS_SHELL, VIS_META,
and VIS_NOLOCALE.

Assorted cleanups and fixes includeing a manpage typo[0].

PR: 210013 [0]
Submitted by: pi [0]

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 after: 2 weeks

Replace our implementation of the vis(3) and unvis(3) APIs with
NetBSD's. This output size limited versions of vis and unvis functions
as well as a set of vis variants that allow arbitrary characters to be
specified for encoding.

Finally, MIME Quoted-Printable encoding as described in RFC 2045 is
supported.