EXPLAIN output now comes out as a query result, not a NOTICE message.

Also, fix debug logging of parse/plan trees so that the messages actually
go through elog(), not directly to stdout.

src/backend/utils/misc/postgresql.conf.sample

@@ -136,6 +136,8 @@
 #debug_print_plan = false
 #debug_pretty_print = false
 
+#explain_pretty_print = true
+
 # requires USE_ASSERT_CHECKING
 #debug_assertions = true