Add module versioning to the passdb module system

Add module versioning to the passdb module system

All passdb modules need to include a 'magic' macro that creates simple
'return my version number' function.

(from metze and jelmer)

Also fix up the dir_drive autosubsitute code to correctly use lp_logon_drive().

(from metze)

Andrew Bartlett
(This used to be commit 4a57c445dd4354034fc41b132a484afe6ab66e16)