better local_hostname default.  According to RFC 2821, it is
recommended that the fqdn hostname be provided in the EHLO/HELO verb
and if that can't be calculated, to use a domain literal.

The rationale for this change is documented in SF patch #497736 which
also had privacy concerns about leaking the fqdn in the EHLO/HELO.  We
decided this wasn't a big concern because no user data is leaked, and
the IP will always be leaked.  The local_hostname argument is provided
for those clients that are super paranoid.

Using localhost.localdomain may break some strict smtp servers so we
decided against using it as the default.