sendmail: do not use IN_CLASS macros

[• karels]

Summary

sendmail was disecting addresses to see if they were on the loopback
"net". Instead, use IN_LOOPBACK(). This is more future-proof.

Test Plan

no functional change

[gshapiro]

If the current code works, I'd prefer to put this change in the upstream first and import it into the FreeBSD source on the next sendmail import. If this is causing breakage now, then we can proceed.

[• karels]

The current code works for now, but there is a proposal for a change that would make the old code fail. Note that IN_LOOPBACK() is not in Linux; not sure about other BSDs. The change could be conditional on #ifdef IN_LOOPBACK.

[gshapiro]

The change (with some minor tweaks) has been committed to the upstream open source distribution and will be part of the next vendor import into FreeBSD.

@karels: An associated release note entry was added, let me know if you would like the attribution modified:

Portability:
        Make use of IN_LOOPBACK, if defined, to determine if
        using a loopback address.  Patch from Mike Karels of
        FreeBSD.

[• karels]

Thanks, sounds good.

[• karels]

Closing review, patch in progress upstream.