Patch from ctrlsoft to make the pluggable passdb subsystem use an lp_list

Patch from ctrlsoft to make the pluggable passdb subsystem use an lp_list
rather than a string when configuring mulitple backends.

Also adjust some of the users of get_global_sam_sid() to cope with the fact
that it just might not exist (uninitialised, can't access secrets.tdb).

More places need conversion.

Add some const and remove silly casts.

Andrew Bartlett
(This used to be commit c264bf2ec93037d2a9927c00295fa60c88b7219d)