mount-util: use mount beneath to replace previous namespace mount

Instead of mounting over, do an atomic swap using mount beneath, if
available. This way assets can be mounted again and again (e.g.:
updates) without leaking mounts.

src/basic/missing_syscall.h

@@ -539,6 +539,10 @@ static inline int missing_open_tree(
 
 /* ======================================================================= */
 
+#ifndef MOVE_MOUNT_BENEATH
+#define MOVE_MOUNT_BENEATH 0x00000200
+#endif
+
 #if !HAVE_MOVE_MOUNT
 
 #ifndef MOVE_MOUNT_F_EMPTY_PATH