Log ALTER SYSTEM statements as DDL

Per discussion in bug #11350, log ALTER SYSTEM commands at the
log_statement=ddl level, rather than at the log_statement=all level.

Pointed out by Tomonari Katsumata.

Back-patch to 9.4 where ALTER SYSTEM was introduced.

src/backend/tcop/utility.c

@@ -2759,7 +2759,7 @@ GetCommandLogLevel(Node *parsetree)
 			break;
 
 		case T_AlterSystemStmt:
-			lev = LOGSTMT_ALL;
+			lev = LOGSTMT_DDL;
 			break;
 
 		case T_VariableSetStmt: