mount-util: make mount_flags_to_string() show flag name instead of number

This also adds missing MS_NOSYMFOLLOW flag. Moreover, this makes the
function always add unhandled flags in number.

src/basic/missing_mount.h

@@ -0,0 +1,8 @@
+#pragma once
+
+#include <sys/mount.h>
+
+/* dab741e0e02bd3c4f5e2e97be74b39df2523fc6e (5.10) */
+#ifndef MS_NOSYMFOLLOW
+#define MS_NOSYMFOLLOW 256
+#endif