config: Consistently truncate value at the first NUL in value input file

When value contained "<file", the file was added as a value. The setting
values don't currently support NULs, so the value gets truncated at the
first NUL. However, the previous code could have appended more data to
the value. This could have produced somewhat confusing results.

src/config/config-parser.c

@@ -479,7 +479,7 @@ str_append_file(string_t *str, const char *key, const char *path,
 		return -1;
 	}
 	while ((ret = read(fd, buf, sizeof(buf))) > 0)
-		str_append_n(str, buf, ret);
+		str_append_data(str, buf, ret);
 	if (ret < 0) {
 		*error_r = t_strdup_printf("%s: read(%s) failed: %m",
 					   key, path);