Make sure to return a proper string in ai_canonname.

Make sure to return a proper string in ai_canonname.

When we made sure that getaddrinfo(3) always resolves "localhost" to
the loopback address we forgot to set ai_canonname if AI_CANONNAME or
AI_FQDN is set. On a successful call ai_canonname has to be a NUL-terminated
string if either of those flags are set.

Problem observed by a@alexis-fouilhe.fr in smtpd(8) with a hostname of
"localhost".

OK millert