Add the pdb_plugin module from Jelmer Vernooij <jelmer@nl.linux.org>.

Add the pdb_plugin module from Jelmer Vernooij <jelmer@nl.linux.org>.

This allow the user to select
'passdb backend = plugin : /path/to/plugin.so : pluging args'

And load any arbitary plugin.  Apparently Jelmer has a mysql plugin in the
works - hence this patch.

We probably need to rework the interface a bit before 3.0 (add versioning of
some kind) but this is a good start.

Andrew Bartlett