Update back-passwd based on patches from howard.chu@platinum.com.

Update back-passwd based on patches from howard.chu@platinum.com.
Modified by me to dn_*() and attr_*() functions and to use
uidObject and to handle search scopes better.  Could use some
more work.  A fun project for anyone wanting to learn how
backends work.
Modified dn_parent and dn_rdn to accept be=NULL (to bypass
be_issuffix check).  These functions should be generalized
and moved to -lldap.  A project for another day.