nvi: fallback to ISO8859-1 as last resort
Current logic of using user's locale encoding that is UTF-8 doesn't make
much sense if we already failed the looks_utf8() check and skipped
encoding set using "fileencoding" as being UTF-8 as well; fallback to
ISO8859-1 in that case.
Reviewed by: Zhihao Yuan <email@example.com>
Differential Revision: https://reviews.freebsd.org/D24919