Remember the source GucContext for each GUC parameter.

We used to just remember the GucSource, but saving GucContext too provides
a little more information --- notably, whether a SET was done by a
superuser or regular user.  This allows us to rip out the fairly dodgy code
that define_custom_variable used to use to try to infer the context to
re-install a pre-existing setting with.  In particular, it now works for
a superuser to SET a extension's SUSET custom variable before loading the
associated extension, because GUC can remember whether the SET was done as
a superuser or not.  The plperl regression tests contain an example where
this is useful.

src/backend/utils/misc/guc-file.l

@@ -296,11 +296,7 @@ ProcessConfigFile(GucContext context)
 								  GUC_ACTION_SET, true);
 		if (scres > 0)
 		{
-			/* variable was updated, so remember the source location */
-			set_config_sourcefile(item->name, item->filename,
-								  item->sourceline);
-
-			/* and log the change if appropriate */
+			/* variable was updated, so log the change if appropriate */
 			if (pre_value)
 			{
 				const char *post_value = GetConfigOption(item->name, true, false);
@@ -315,7 +311,17 @@ ProcessConfigFile(GucContext context)
 		}
 		else if (scres == 0)
 			error = true;
-		/* else no error but variable was not changed, do nothing */
+		/* else no error but variable's active value was not changed */
+
+		/*
+		 * We should update source location unless there was an error, since
+		 * even if the active value didn't change, the reset value might have.
+		 * (In the postmaster, there won't be a difference, but it does matter
+		 * in backends.)
+		 */
+		if (scres != 0)
+			set_config_sourcefile(item->name, item->filename,
+								  item->sourceline);
 
 		if (pre_value)
 			pfree(pre_value);