bug #34372: filter out dead remote hosts before running on remotes

[Ole Tange]

Basic --filter-hosts is now working in git version.

  parallel  --filter-hosts -S server-a -S nonexistant -S
existant-but-down --onall hostname\;echo ::: a b

This will remove nonexistant and existant-but-down from the list of
servers and only run commands on server-a.

Currently you can NOT put --filter-hosts in a profile, $PARALLEL,
/etc/parallel/config or similar. This is because GNU Parallel uses GNU
Parallel to compute this, so you will get an infinite loop.

Feed back wanted.


/Ole
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