smtpd.go: include the TLS protocol version in our TLS log line

I have also changed the log line to not mention the server name if
it's blank, which it almost always is. A blank server name is not
interesting.
sinksmtp: rethink TLS protocol version handling

Now we log the protocol version that was negotiated but do not insist on
a minimum version. The handling we may want for minimum TLS versions and
so on is complicated enough that it should be pushed to the configuration
language, and in the mean time having SSLv3 beats not having SSL at all.
sinksmtp: stop checking (and thus asking for) TLS client certificates

Setting tls.VerifyClientCertIfGiven apparently has side effects on the
protocol flow that crypto.tls expects for SSLv3 connections, so that
clients without a certificate will fail the SSL connection setup. After
mulling it over, I've decided that this is undesirable in general so
now sinksmtp doesn't ever set this option, not even on a first connection.

It was interesting to find out how many clients offered bad certificates
(although I never measured how many offered good ones), but not as
interesting as having SSLv3 connections.