Issue 551: Fix the best-effort UTF8 conversion

Issue 551:  Fix the best-effort UTF8 conversion

If a valid character set conversion is impossible, the code falls back
to a best-effort conversion that preserves ASCII bytes and converts
the rest to Unicode Replacement Characters (if the output is UTF8)
or '?' (otherwise).  This code did not correctly track the remaining
bytes in the output buffer; I've replaced this with slower and simpler
code that utilizes the safe string append functions.