cmd/tailscale: add "debug via" subcommand to do CIDR math for via ranges

$ tailscale debug via 0xb 10.2.0.0/16
fd7a:115c:a1e0:b1a:0:b:a02:0/112
$ tailscale debug via fd7a:115c:a1e0:b1a:0:b:a02:0/112
site 11 (0xb), 10.2.0.0/16

Previously: 3ae701f0eb

This adds a little debug tool to do CIDR math to make converting between
those ranges easier for now.

Updates #3616

Change-Id: I98302e95d17765bfaced3ecbb71cbd43e84bff46
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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