Solved !
With some professional nudging it turns out that the "subliminal" messages
that have been appearing during shutdown are mostly the output murmurings of
applications that were somehow started by the Boot-Up Manager (bum) and which
I wasn't using. I've already mentioned apache2 and related software (which I
purged) ... the others are used occasionally but are not needed during
boot-up.
Unchecking those applications in Boot-Up Manager doesn't banish them, but it
at least discourages them from piping up during shutdown.
For example, I really do want ClamAV and FreshClam to be running, and when I
have been moving large files around in Icedove, enough memory is used that
the shutdown process is slowed down sufficiently that I can actually read the
remaining messages put into that momentary white-text-on-black screen: They
are from ClamAv and its daemon, etc. !
In the words of that famous psychiatric patient: after great expenditures of
time and money, whenever the surreptitious messages appear during shutdown,
they don't bother me any more.
