derp/derphttp: strip port numbers from URL hostname

When trying to set up multiple derper instances meshing with each
other, it turned out that while one can specify an alternative
listening port using the -a flag, the TLS hostname gets incorrectly
determined and includes the set alternative listening port as part of
the hostname. Thus, the TLS hostname validation always fails when the
-mesh-with values have ports.

Updates #9949

Signed-off-by: Thomas Kosiewski <thomas.kosiewski@loft.sh>

derp/derphttp/derphttp_client.go

@@ -226,7 +226,7 @@ func (c *Client) useHTTPS() bool {
 // tlsServerName returns the tls.Config.ServerName value (for the TLS ClientHello).
 func (c *Client) tlsServerName(node *tailcfg.DERPNode) string {
 	if c.url != nil {
-		return c.url.Host
+		return c.url.Hostname()
 	}
 	return node.HostName
 }