explain refactoring v4

OK, here it is again. Changes are the same as the previous version,
but this one should apply cleanly after today's pgindent run.

http://archives.postgresql.org/message-id/603c8f070906051803nd4a9e3ekb72bcc379ad73f16@mail.gmail.com

(Rebasing this wasn't too bad, but I have to say that one thing that's
frustrating is that there's no obvious rule for how continuation lines
should be indented. If it were easy for patch authors and committers
to understand what to do, pgindent would change less stuff, which
would make for less rebasing. Just to take one example, in explain.h,
the level of hanging indent for the parameters of ExplainOnePlan is
different than the level for ExplainPrintPlan. That's presumably
because ExplainPrintPlan is two characters longer than ExplainOnePlan,
but beyond that I'm lost.)

...Robert