helo: added IPv6 support to invalid_localhost

and also suppress a test warning
added Utils::is_localhost and t/qpsmtpd-utils.t

resolves Issue #89
Merge pull request #90 from msimerson/utils

Utils: added is_localhost and tests.
2x: use Utils->is_localhost() to detect loopback

* it's IPv6 compatible
* plugins/helo, plugins/fcrdns