strvis(3): Avoid internal state of multibyte functions being tainted.

The mbtoc(3) and wctomb(3) functions use internal state which may be
tainted before the call to strvis(3). In this context we can just use
the thread-safe versions mbrtoc(3) and wcrtomb(3) which allow passing
our own state from our stack.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon