crypto/tls: add server side SNI support.

With this in place, a TLS server is capable of selecting the correct
certificate based on the client's ServerNameIndication extension.

The need to call Config.BuildNameToCertificate is unfortunate, but
adding a sync.Once to the Config structure made it uncopyable and I
felt that was too high a price to pay. Parsing the leaf certificates
in each handshake was too inefficient to consider.

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5151048

src/pkg/crypto/tls/handshake_server.go

@@ -115,7 +115,12 @@ FindCipherSuite:
 	}
 
 	certMsg := new(certificateMsg)
-	certMsg.certificates = config.Certificates[0].Certificate
+	if len(clientHello.serverName) > 0 {
+		c.serverName = clientHello.serverName
+		certMsg.certificates = config.getCertificateForName(clientHello.serverName).Certificate
+	} else {
+		certMsg.certificates = config.Certificates[0].Certificate
+	}
 	finishedHash.Write(certMsg.marshal())
 	c.writeRecord(recordTypeHandshake, certMsg.marshal())