HEADS UP: No longer need to recompile milters when upgrading

[Gregory Shapiro]

The libmilter ABI breakage which required recompiling mail filters
(milters) has been fixed in the HEAD (CURRENT).  The change will be
MFC'ed after the customary 3-4 days to the RELENG_[456] branches.

It is no longer necessary to recompile mail filters compiled against
an older libmilter.so shared library.  Additionally, if you did recompile
them already, you do not need to recompile them again.