s3:trusts_util: generate completely random passwords in trust_pw_change()

s3:trusts_util: generate completely random passwords in trust_pw_change()

Instead of having every 2nd byte as '\0' in the utf16 password,
because the utf8 form is based on an ascii subset, we convert
the random buffer from CH_UTF16MUNGED to CH_UTF8.

This way we have a random but valid utf8 string,
which is almost like what Windows is doing.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>