netbsd-tests: libc: fix strvis(3) overflow tests

These tests weren't run on x86 until CI grabbed them. It turns out,
there's a sign extension bug that surfaces on x86 with char being a
signed type.

NetBSD unearthed this when they took and improved the patch, so just
grab their solution until we get to merging in the latest version of
the test.

Reported by: CI (via ngie)
Fixes: 2f489a509e61 ("libc: fix some overflow scenarios in vis(3)")

libc: fix some overflow scenarios in vis(3)

The previous incarnation of this would call wcrtomb() on the destination
buffer, and only check for overflow *after* it's happened.
Additionally, the conversion error / VIS_NOLOCALE path also didn't check
for overflow, and the overflow check at the end didn't account for the
fact that we still need to write a NUL terminator afterward.

Start by only doing the multibyte conversion into mbdst directly if we
have enough buffer space to guarantee it'll fit. An additional
MB_CUR_MAX buffer has been stashed on the stack to write into if we're
cutting it close at the end of the buffer, since we don't really have a
good way to determine the length of the wchar_t without just doing the
conversion. We'll do the conversion into the buffer that's guaranteed
to fit, then copy it over if the copy won't overflow.

The byte-for-byte overflow is a little bit easier, as we simply check
for overflow with each byte written and make sure we can still NUL
terminate after.

Tests added to exercise these edge cases.

Reviewed by: des
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D41328

Don't build :strvis_locale if VIS_NOLOCALE is undefined

The copy of contrib/libc-vis on ^/stable/10 doesn't contain all of the features
in the ^/stable/11 // ^/head version, including VIS_NOLOCALE. The risk is lower
in conditionally running the test instead of backporting the newer version of
libc-vis

MFC after: now