Make pdb_tdb honour a private dir overridden in smb.conf.

Make pdb_tdb honour a private dir overridden in smb.conf.

One lp_private_dir() has to be used instead of get_dyn_PRIVATE_DIR()
to determine the location of the passdb.tdb.

I noticed this when running make test as a "normal user" from a
build, where I had done "make install" as root before, and so
the passdb.tdb could not be accessed during the startup phase
"CREATE TEST ENVIRONMENT IN ./st ..." in selftest.sh.

Michael
(This used to be commit 1f96389afa7250af7393489fb538b8aed93d815c)