Use appendStringInfoString/Char et al where appropriate.

Patch by David Rowley. Backpatch to 9.5, as some of the calls were new in
9.5, and keeping the code in sync with master makes future backpatching
easier.

contrib/postgres_fdw/postgres_fdw.c

@@ -2738,7 +2738,7 @@ postgresImportForeignSchema(ImportForeignSchemaStmt *stmt, Oid serverOid)
 		}
 
 		/* Append ORDER BY at the end of query to ensure output ordering */
-		appendStringInfo(&buf, " ORDER BY c.relname, a.attnum");
+		appendStringInfoString(&buf, " ORDER BY c.relname, a.attnum");
 
 		/* Fetch the data */
 		res = PQexec(conn, buf.data);