Export ExplainBeginOutput() and ExplainEndOutput() for auto_explain.

Without these functions, anyone outside of explain.c can't actually use
ExplainPrintPlan, because the ExplainState won't be initialized properly.
The user-visible result of this was a crash when using auto_explain with
the JSON output format.

Report by Euler Taveira de Oliveira.  Analysis by Tom Lane.  Patch by me.