net/dns: add Plan 9 support

This requires the rsc/plan9 ndb DNS changes for now:

https://9fans.topicbox.com/groups/9fans/T9c9d81b5801a0820/ndb-suffix-specific-dns-changes
https://github.com/rsc/plan9/commit/e8c148ff092a5780d04aa2fd4a07a5732207b698
https://github.com/rsc/plan9/commit/1d0642ae493bf5ce798a6aa64a745bc6316baa11

Updates #5794

Change-Id: I0e242c1fe7bb4404e23604e03a31f89f0d18e70d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f