idn_locale_to_ace() is a static function which is always used with a
buffer of size MXNAME, i.e. one that can fit any valid domain name.
Since libidn2 detects invalid domain names and libidn2 errors are
considered fatal, remove size checks from idn_locale_to_ace().  This
makes returning an isc_result_t from it unnecessary.

Do not process success cases in conditional branches for improved
consistency with the rest of BIND source code.  Add a comment explaining
the purpose of idn_locale_to_ace().  Rename that function's parameters
to match common BIND naming pattern.