This is a port of commit e3f3054f8b74caa639a6d9be09702693af9a70e7 from
meek.

In the previous commit, we changed from separate Listen and Serve steps
to always calling ListenAndServe. However, we would really like to
immediately get feedback if any errors happen in the Listen step inside
the call, because it's much better for debugging if those errors get
reported to tor through SMETHOD-ERROR--rather than reporting success to
tor and actually logging an error only in the snowflake log. So we wait
100 ms for an error to occur before deciding that the Listen succeeded.

We don't need to apply this hack to the ACME HTTP-01 listener, because
it's a plaintext listener. Unlike in the TLS case, there isn't any
internal magic that the net library does that we have to rely on. We
just call net.ListenTCP and check for an error.