r11536: Add a hook for client-principal access control to hdb-ldb, re-using

r11536: Add a hook for client-principal access control to hdb-ldb, re-using
the code in auth/auth_sam.c for consistancy.  This will also allow us
to have one place for a backend directory hook.

I will use a very similar hook to add the PAC.

Andrew Bartlett
(This used to be commit 4315836cd8c94eb8340c4050804face4d0066810)