wol-f3s: umount NFS filesystems on earth before shutdown

Before shutting down f3s Beelink hosts (f0/f1/f2/f3), attempt to
umount all NFS filesystems currently mounted on earth. NFS mounts on
earth go through stunnel (127.0.0.1:2323 -> 192.168.1.138:2323 CARP
VIP on f0/f1). If any mount is active and cannot be umounted, the
shutdown is aborted to prevent data loss or a hung filesystem.

- Add umount_nfs_mounts() that reads /proc/mounts for nfs/nfs4 types,
  attempts to umount each, and returns 1 if any fail
- Call umount_nfs_mounts before proceeding in shutdown, shutdown-f3,
  and shutdown-all cases (shutdown-pis skipped: Pis do not serve NFS)
- Update all comments to explain the NFS-over-stunnel architecture and
  the safety rationale

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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