fix bugs in ping logic, ping hosts in parallel, and tweak README and LICENSE

The ping logic had some bugs in the wait logic which are now fixed.

Pinging hosts in parallel will speed up CI.

LICENSE and README got small updates in preparation for moving this code
to tailscale/github-action.

Updates tailscale/corp#32862

Signed-off-by: Percy Wegmann <percy@tailscale.com>