core: hook up MountFlags= to the transient unit logic

This makes "systemd-run -p MountFlags=shared -t /bin/sh" work, by making
MountFlags= to the list of properties that may be accessed transiently.

src/basic/mount-util.h

@@ -61,3 +61,6 @@ int mount_verbose(
                 unsigned long flags,
                 const char *options);
 int umount_verbose(const char *where);
+
+const char *mount_propagation_flags_to_string(unsigned long flags);
+unsigned long mount_propagation_flags_from_string(const char *name);