unicode: simplify utf8len

Just use the utf8nlen implementation with a (size_t)-1 len argument,
similar to utf8_lookup.  Also move the function to utf8-selftest.c, as
it isn't used anywhere else.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>