Move milter state creation from connect callback to envfrom

[rhansen]

If IPv6 support is not enabled in libmilter, the connect callback is not called when an IPv6 client connects. Work around the issue by moving state initialization from the connect callback to the envelope-from callback.

Note that the state is now per-message, not per-connection, so it must be cleaned up in the end-of-message callback, in the abort callback, and in any other callback that finalizes message handling (e.g., tempfail).

Fixes #156

[roehling]

Thanks. This is probably a better solution than my initial idea of patching IPv6 support in libmilter.