Include the target table in EXPLAIN output for ModifyTable nodes.

Per discussion, this seems important for plans involving writable CTEs,
since there can now be more than one ModifyTable node in the plan.

To retain the same formatting as for target tables of scan nodes, we
show only one target table, which will be the parent table in case of
an UPDATE or DELETE on an inheritance tree.  Individual child tables
can be determined by inspecting the child plan trees if needed.