tor-browser, 60-tor-ready.sh: use systemctl to determine if Tor has bootstrapped already.

Note that `systemctl is-active' returns false for a service that's in
"activating" state. The thing is, tails-wait-until-tor-has-bootstrapped.service
*is* in "activating" state until Tor has bootstrapped. So, we explicitly check
for its state to be "inactive" (meaning it has completed, and Tor has
bootstrapped).