Now that PAM itself is multiarch-aware, I'm looking at converting my
PAM modules to also be multiarch, but there doesn't appear to be any
documentation and the MiniHowto hasn't been updated.

Also, it looks like multiarch support was added to libpam0g's shared
library (to look for modules in the appropriate directory), but there was
no shlibs bump, which means that modules have to force a dependency on a
new enough version manually.  While this isn't exactly what the shlibs
system is intended for (since the symbols in the shared library haven't
really changed), I think bumping shlibs will probably produce better
results for the archive as a whole once people start updating PAM
modules.

I'm converting libpam-afs-session as an experiment.  Here's what I've
done, for reference (and hopeful help in updating the documentation):

* Follow the general multiarch instructions for shared libraries.  I
  suspect some parts of this aren't actually necessary, such as the
  pre-dependency on multiarch-support, but it doesn't seem like it would
  hurt and I wasn't sure.

* Install the module in /lib/$(DEB_HOST_MULTIARCH)/security.

* Add a Depends on libpam0g (>= 1.1.3-2~) to ensure that the PAM library
  knows to look for modules in that path.