u9fs: unstable-2020-11-21 -> unstable-2021-25
u9fs: unstable-2020-11-21 -> unstable-2021-25

* Use new (?) upstream over disappeared 9front hg repo, see also
  https://www.mail-archive.com/9fans@9fans.net/msg39324.html

* Link to Plan 9 Foundation's man page viewer as the Bell Labs has gone
  offine.

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