packages: fail fast on empty DTail version (review follow-up)

- Makefile: new check-dtail-version target guards dtail-openbsd,
  dtail-freebsd, and the build-VM stamp recipe — a wrong DTAIL_SRC now
  aborts with a clear error before any VM start, /tmp cleanup, or
  remote work runs with an empty version in the file names (dtail-netbsd
  already had an equivalent inline guard)
- Makefile: dtail-openbsd pre-cleans its stale staging tgz like the
  netbsd target, so a failed run can never silently upload an old
  package
- pkg-dtail-openbsd.sh / pkg-dtail-freebsd.sh: reject a missing version
  argument like pkg-dtail-netbsd.sh (would otherwise build dtail-.tgz)

Verified: make -n renders the guard first for both targets;
make dtail-{openbsd,freebsd} DTAIL_SRC=/nonexistent fails fast with the
version error before any rm/remote step. Guards only change failure
paths — no package rebuild needed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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