The relevant standards allow the mbtowc/mbrtowc functions to reject
non-ASCII characters (i.e., chars with the high bit set) when the locale
is C or POSIX. The BSD libraries (e.g., on OS X) don't do this but
the GNU libraries (e.g., on Linux) do. Like most programs we need the
C/POSIX locales to allow arbitrary bytes. So explicitly check if we're
in a single-byte locale (which would also include ISO-8859 variants)
and simply pass-thru the chars without encoding or decoding.
Fixes#2802.
