fix: don't listen on every interface in createTunnel

[VojtaM39]

createTunnel() called server.listen(0) without a host, so the tunnel bound the
unspecified address (:: / 0.0.0.0). Any peer that could reach the host could tunnel
to targetHost using those credentials.

GHSA-5vwf-g8jp-pgj3, #660.

Changes:

• createTunnel() listens on 127.0.0.1 by default. The port and hostname options
  documented in the README work again — they were dropped from the implementation in v2.0.0.
  Pass hostname: '0.0.0.0' to opt back into the old binding.
• Warns when bound to a non-loopback address.
• Tests for the listener address.

Behavior change: the returned endpoint is 127.0.0.1:<port> instead of 0.0.0.0:<port>.

I have also pushed a branch tmp/tunnel-connect-fix-with-repro with repro, that demonstrates that the fix works.