Revert "shutdown: do not umount recursively before MS_MOVE"

This reverts commit 6b219b74de.

This commit doesn't look right to me. We have to unmount everything
recursively *before* we MS_MOVE because the MS_MOVE will not get rid of
it for us, and we simply cannot access these mounts after the MS_MOVE is
complete anymore.

This is a fundamental difference between MS_MOVE and pivot_root(). The
latter repivots the entire mount table getting rid of anything outside
of the new root. MS_MOVE otoh just mounts a bunch of mount points to the
top, leaving in place whatever might be underneath it.

Thus, if we go through the MS_MOVE codepath we must unmount everything
explicitly before doing so because otherwise the mounts will be pinned
forever, but be entirely invisble to userspace.

src/shared/switch-root.h

@@ -8,7 +8,6 @@ typedef enum SwitchRootFlags {
                                                      * that it is backed by non-persistent tmpfs/ramfs/… */
         SWITCH_ROOT_DONT_SYNC             = 1 << 1, /* don't call sync() immediately before switching root */
         SWITCH_ROOT_RECURSIVE_RUN         = 1 << 2, /* move /run/ with MS_REC from old to new root */
-        SWITCH_ROOT_SKIP_RECURSIVE_UMOUNT = 1 << 3, /* do not umount recursively on move */
 } SwitchRootFlags;
 
 int switch_root(const char *new_root, const char *old_root_after, SwitchRootFlags flags);