Introduce a new `tailscale routecheck` command which prints a report
of high-availability routers that are reachable.
This command rhymes with the `tailscale netcheck` command and but
instead of reporting on local network conditions, `routecheck` reports
on remote connectivity.
Updates #17366
Updates tailscale/corp#33033
Signed-off-by: Simon Law <sfllaw@tailscale.com>
2d6844c565