Add function next_mb_char_size() that returns a size_t of the number of

Add function next_mb_char_size() that returns a size_t of the number of
bytes in the mb character at a pointer. Will be useful in fixing check_path_syntax()
to not use a "blacklist". Also re-added my (C) to reply.c. I mean, really - I've
been adding code to the file for over 10 years and I recognise many of the
fuctions as mine ! :-).
Jeremy.
(This used to be commit d2b2a39fd2f2e06cc4aebf081876985f0b28b477)