Converts @hostname to its canonical presentation form; a UTF-8
string in Unicode normalization form C, containing no uppercase
letters, no forbidden characters, and no ASCII-encoded segments,
and not ending with a trailing dot.
Of course if @hostname is not an internationalized hostname, then
the canonical presentation form will be entirely ASCII.
Converts @hostname to its canonical presentation form; a UTF-8 string in Unicode normalization form C, containing no uppercase letters, no forbidden characters, and no ASCII-encoded segments, and not ending with a trailing dot.
Of course if @hostname is not an internationalized hostname, then the canonical presentation form will be entirely ASCII.