Add is_utf8_valid_partial_char()

Add is_utf8_valid_partial_char()

This new function can test some purported UTF-8 to see if it is
well-formed as far as it goes.  That is there aren't enough bytes for
the character they start, but what is there is legal so far.  This can
be useful in a fixed width buffer, where the final character is split in
the middle, and we want to test without waiting for the next read that
the entire buffer is valid.