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

mox-/config.go

@@ -929,6 +929,19 @@ func PrepareStaticConfig(ctx context.Context, log mlog.Log, configFile string, c
 		}
 	}
 
+	checkTransportDirect := func(name string, t *config.TransportDirect) {
+		if t.DisableIPv4 && t.DisableIPv6 {
+			addErrorf("transport %s: both IPv4 and IPv6 are disabled, enable at least one", name)
+		}
+		t.IPFamily = "ip"
+		if t.DisableIPv4 {
+			t.IPFamily = "ip6"
+		}
+		if t.DisableIPv6 {
+			t.IPFamily = "ip4"
+		}
+	}
+
 	for name, t := range c.Transports {
 		n := 0
 		if t.Submissions != nil {
@@ -947,6 +960,10 @@ func PrepareStaticConfig(ctx context.Context, log mlog.Log, configFile string, c
 			n++
 			checkTransportSocks(name, t.Socks)
 		}
+		if t.Direct != nil {
+			n++
+			checkTransportDirect(name, t.Direct)
+		}
 		if n > 1 {
 			addErrorf("transport %s: cannot have multiple methods in a transport", name)
 		}