Split VFS.MountAt into VFS.MountDisconnected and VFS.ConnectMountAt.

This makes it straightforward to create bind mounts internally in VFS2: Given a
bind mount root represented by vfs.VirtualDentry vd:

- Create a new mount with VFS.NewDisconnectedMount(vd.Mount().Filesystem(),
  vd.Dentry()).

- Connect the resulting mount in the appropriate namespace with
  VFS.ConnectMountAt().

Note that the resulting bind mount is non-recursive; recursive bind mounting
requires explicitly duplicating all children of the original mount, which is
best handled internally by VFS.

Updates #179

PiperOrigin-RevId: 313703963

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