Don't use 'bool' as a struct member name in help_config.c.

Doing so doesn't work if bool is a macro rather than a typedef.

Although c.h spends some effort to support configurations where bool is
a preexisting macro, help_config.c has existed this way since
2003 (b700a6), and there have not been any reports of
problems. Backpatch anyway since this is as riskless as it gets.

Discussion: 20150812084351.GD8470@awork2.anarazel.de
Backpatch: 9.0-master

src/backend/utils/misc/help_config.c

@@ -31,7 +31,7 @@
 typedef union
 {
 	struct config_generic generic;
-	struct config_bool bool;
+	struct config_bool _bool;
 	struct config_real real;
 	struct config_int integer;
 	struct config_string string;
@@ -98,7 +98,7 @@ printMixedStruct(mixedStruct *structToPrint)
 
 		case PGC_BOOL:
 			printf("BOOLEAN\t%s\t\t\t",
-				   (structToPrint->bool.reset_val == 0) ?
+				   (structToPrint->_bool.reset_val == 0) ?
 				   "FALSE" : "TRUE");
 			break;