[Mimedefang] Adding filter_helo support

[Philip Prindeville]

Quick question...

I was wondering about moving the:

    write_mx_command(data->cmdFD, 'E', (unsigned char *) data->heloArg);

into the helo() function from envfrom() instead...  but I'm not sure at 
what point
the file is opened and can be written to...  currently that happens in 
envfrom()
too, but...  it's not clear what stuff that can't happen in mfconnect 
because it's
too early can happen in helo instead of envfrom().

Also, what about including write_macro_value() calls for:

{tls_version}, {cipher}, {cipher_bits}, {cert_subject}, {cert_issuer}

since this might be of interest in filter_helo() as well.

Thanks,

-Philip