No warning about the need of change the location of milter blacklist files

When updating the postfix package (MRE, LP: #1995312), Scott K.
advised of a bug reported in Debian that would affect such an upgrade.

The following change introduced the modified behavior:

    - Cleanup (problem introduced: Postfix 2.7): milter_header_checks
      maps are now opened before the cleanup server enters the
      chroot jail.

This could lead to breaking some configurations ( broken incoming mail) because a milter blacklist file that used to have to be in the chroot directory (/var/spool/postfix) is no longer there and needs to be moved to /.

I have weighed whether to try to revert that change fully or partially, but I have made several unsuccessful attempts without reintroducing the bug that this change removed. As there is a workaround/new way of usage ( moving the relative files to /), I think we can do the same as Debian and wait for the fix/notification of the new way of usage in an upcoming stable release of postfix.

I file this bug to keep track of the Debian bug and let users who may face it know we are aware of it. Anyway, if anyone thinks it needs to be reported upstream, please feel free to communicate it there.