add TransportDirect transport

The `TransportDirect` transport allows to tweak outgoing SMTP
connections to remote servers. Currently, it only allows to select
network IP family (ipv4, ipv6 or both).

For example, to disable ipv6 for all outgoing SMTP connections:
- add these lines in mox.conf to create a new transport named
"disableipv6":
```
Transports:
  disableipv6:
    Direct:
      DisableIpv6: true
```
- then add these lines in domains.conf to use this transport:
```
Routes:
  -
    Transport: disableipv6
```

fix #149

webmail/api.go

@@ -1732,7 +1732,7 @@ func recipientSecurity(ctx context.Context, resolver dns.Resolver, messageAddres
 
 		// Resolve the IPs. Required for DANE to prevent bad DNS servers from causing an
 		// error result instead of no-DANE result.
-		authentic, expandedAuthentic, expandedHost, _, _, err := smtpclient.GatherIPs(ctx, log.Logger, resolver, host, map[string][]net.IP{})
+		authentic, expandedAuthentic, expandedHost, _, _, err := smtpclient.GatherIPs(ctx, log.Logger, resolver, "ip", host, map[string][]net.IP{})
 		if err != nil {
 			rs.DANE = SecurityResultError
 			return