Implement couriers

This patch introduces the couriers, which the queue uses to deliver mail.

We have a local courier (using procmail), a remote courier (uses SMTP), and a
router courier that decides which of the two to use based on a list of local
domains.

There are still a few things pending, but they all have their basic
functionality working and tested.

internal/courier/courier.go

@@ -0,0 +1,48 @@
+// Package courier implements various couriers for delivering messages.
+package courier
+
+import "strings"
+
+// Courier delivers mail to a single recipient.
+// It is implemented by different couriers, for both local and remote
+// recipients.
+type Courier interface {
+	Deliver(from string, to string, data []byte) error
+}
+
+// Router decides if the destination is local or remote, and delivers
+// accordingly.
+type Router struct {
+	Local        Courier
+	Remote       Courier
+	LocalDomains map[string]bool
+}
+
+func (r *Router) Deliver(from string, to string, data []byte) error {
+	d := domainOf(to)
+	if r.LocalDomains[d] {
+		return r.Local.Deliver(from, to, data)
+	} else {
+		return r.Remote.Deliver(from, to, data)
+	}
+}
+
+// Split an user@domain address into user and domain.
+func split(addr string) (string, string) {
+	ps := strings.SplitN(addr, "@", 2)
+	if len(ps) != 2 {
+		return addr, ""
+	}
+
+	return ps[0], ps[1]
+}
+
+func userOf(addr string) string {
+	user, _ := split(addr)
+	return user
+}
+
+func domainOf(addr string) string {
+	_, domain := split(addr)
+	return domain
+}