Add SETTINGS option to EXPLAIN, to print modified settings.

Query planning is affected by a number of configuration options, and it
may be crucial to know which of those options were set to non-default
values.  With this patch you can say EXPLAIN (SETTINGS ON) to include
that information in the query plan.  Only options affecting planning,
with values different from the built-in default are printed.

This patch also adds auto_explain.log_settings option, providing the
same capability in auto_explain module.

Author: Tomas Vondra
Reviewed-by: Rafia Sabih, John Naylor
Discussion: https://postgr.es/m/e1791b4c-df9c-be02-edc5-7c8874944be0@2ndquadrant.com

src/include/commands/explain.h

@@ -35,6 +35,7 @@ typedef struct ExplainState
 	bool		buffers;		/* print buffer usage */
 	bool		timing;			/* print detailed node timing */
 	bool		summary;		/* print total planning and execution timing */
+	bool		settings;		/* print modified settings */
 	ExplainFormat format;		/* output format */
 	/* state for output formatting --- not reset for each new plan tree */
 	int			indent;			/* current indentation level */