For 2.1.8 (this task), deprecate (via doxygen comments) direct access to all members of struct pe_working_set_s.

We will need new public APIs that SBD can be updated to use instead of direct access (T794). We will need to keep the members SBD uses around until we no longer support any OSes shipping an older version.

We need new public APIs to:

• set input
• get dc_node
• get no_quorum_policy
• find a node (equivalent of pe_find_node(scheduler->nodes, NAME))
• check whether cluster is quorate (equivalent of pcmk_is_set(data_set->flags, pcmk_sched_quorate))

For 3.0.0 (T798), we can rename it to struct pcmk__scheduler, and replace all members except the above with a void *private member pointing to a new internal struct for the replaced members.