Assorted minor refactoring in EXPLAIN.

This is believed to not change the output at all, with one known exception:
"Subquery Scan foo" becomes "Subquery Scan on foo".  (We can fix that if
anyone complains, but it would be a wart, because the old code was clearly
inconsistent.)  The main intention is to remove duplicate coding and
provide a cleaner base for subsequent EXPLAIN patching.

Robert Haas