Update sepgsql to add mandatory access control for TRUNCATE

Use SELinux "db_table: { truncate }" to check if permission is granted to
TRUNCATE. Update example SELinux policy to grant needed permission for
TRUNCATE. Add new regression test to demonstrate a positive and negative
cases. Test will only be run if the loaded SELinux policy has the
"db_table: { truncate }" permission. Makes use of recent commit which added
object TRUNCATE hook. Patch by Yuli Khodorkovskiy with minor
editorialization by me. Not back-patched because the object TRUNCATE hook
was not.

Author: Yuli Khodorkovskiy
Reviewed-by: Joe Conway
Discussion: https://postgr.es/m/CAFL5wJcomybj1Xdw7qWmPJRpGuFukKgNrDb6uVBaCMgYS9dkaA%40mail.gmail.com

contrib/sepgsql/hooks.c

@@ -188,6 +188,20 @@ sepgsql_object_access(ObjectAccessType access,
 			}
 			break;
 
+		case OAT_TRUNCATE:
+			{
+				switch (classId)
+				{
+					case RelationRelationId:
+						sepgsql_relation_truncate(objectId);
+						break;
+					default:
+						/* Ignore unsupported object classes */
+						break;
+				}
+			}
+			break;
+
 		case OAT_POST_ALTER:
 			{
 				ObjectAccessPostAlter *pa_arg = arg;