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