Refactor: libpacemaker: add new file pcmk_sched_fencing.c

... starting with just pcmk__order_vs_fence() from pcmk_graph_producer.c, which
doesn't have anything to do with producing transition graphs.

The intent is that the file will have code related to the scheduling of
fencing, as opposed to the high-level functions for actually performing fencing
in pcmk_fence.c.