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)`)
* indicate the cluster has a fencing device (equivalent of `pcmk__set_scheduler_flags(scheduler, pcmk_sched_have_fencing)`)
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.