Rename session_auth_is_superuser to current_role_is_superuser.

This variable might've been accurately named when it was added in
ea886339b8, but the name hasn't been accurate since at least the
introduction of SET ROLE in e5d6b91220.  The corresponding
documentation was fixed in eedb068c0a.  This commit renames the
variable accordingly.

Suggested-by: Joseph Koshakow
Discussion: https://postgr.es/m/CAAvxfHc-HHzONQ2oXdvhFF9ayRnidPwK%2BfVBhRzaBWYYLVQL-g%40mail.gmail.com

src/backend/access/transam/parallel.c

@@ -327,7 +327,7 @@ InitializeParallelDSM(ParallelContext *pcxt)
 	fps->database_id = MyDatabaseId;
 	fps->authenticated_user_id = GetAuthenticatedUserId();
 	fps->outer_user_id = GetCurrentRoleId();
-	fps->is_superuser = session_auth_is_superuser;
+	fps->is_superuser = current_role_is_superuser;
 	GetUserIdAndSecContext(&fps->current_user_id, &fps->sec_context);
 	GetTempNamespaceState(&fps->temp_namespace_id,
 						  &fps->temp_toast_namespace_id);

src/backend/utils/misc/guc_tables.c

@@ -511,7 +511,7 @@ bool		check_function_bodies = true;
  * details.
  */
 bool		default_with_oids = false;
-bool		session_auth_is_superuser;
+bool		current_role_is_superuser;
 
 int			log_min_error_statement = ERROR;
 int			log_min_messages = WARNING;
@@ -1037,13 +1037,16 @@ struct config_bool ConfigureNamesBool[] =
 		NULL, NULL, NULL
 	},
 	{
-		/* Not for general use --- used by SET SESSION AUTHORIZATION */
+		/*
+		 * Not for general use --- used by SET SESSION AUTHORIZATION and SET
+		 * ROLE
+		 */
 		{"is_superuser", PGC_INTERNAL, UNGROUPED,
 			gettext_noop("Shows whether the current user is a superuser."),
 			NULL,
 			GUC_REPORT | GUC_NO_SHOW_ALL | GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
 		},
-		&session_auth_is_superuser,
+		&current_role_is_superuser,
 		false,
 		NULL, NULL, NULL
 	},

src/include/utils/guc.h

@@ -250,7 +250,7 @@ extern PGDLLIMPORT bool log_statement_stats;
 extern PGDLLIMPORT bool log_btree_build_stats;
 
 extern PGDLLIMPORT bool check_function_bodies;
-extern PGDLLIMPORT bool session_auth_is_superuser;
+extern PGDLLIMPORT bool current_role_is_superuser;
 
 extern PGDLLIMPORT bool log_duration;
 extern PGDLLIMPORT int log_parameter_max_length;