all_string_sub() is broken as it doesn't handle mb chars correctly (and

all_string_sub() is broken as it doesn't handle mb chars correctly (and
so breaks when substitution '/' and '\'). It's used by unix_clean_name(),
which is used by reduce_name, which is used by check_name() (phew!).
Now that we know all filenames passed to check_name() are in a "good"
format (no double slashes, all '\\' chars translated to '/' etc.) due
to the new check_path_syntax() we can avoid calling reduce_name unless
widelinks are denied. After this check-in I can fix all_string_sub() to
handle mb chars correctly as it won't be in the direct path in the
main path handling code.
Jeremy.
(This used to be commit 6080186fc4c2e7c59dd12a177539bfb77eb525cb)