policy.TLS_FORWARD: send SNI on wire if configured

In https world it's standard to do that, and it's relied on.
Real-life example: 8.8.8.8#853 over TLSv1.3 won't send a certificate
if we don't send SNI (no idea why; also they do send it with TLSv1.2).

As a consequence, we no longer allow multiple hostnames per
address-port tuple, but that didn't seem useful.