Params: change UTF8 construct calls to avoid explicit strlen(3) calls.

Params: change UTF8 construct calls to avoid explicit strlen(3) calls.

It is better, safer and smaller to let the library routine handle the
strlen(3) call.

Added a note to the documentation suggesting this.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11019)