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/procmail_test.go

@@ -0,0 +1,65 @@
+package courier
+
+import (
+	"bytes"
+	"io/ioutil"
+	"os"
+	"testing"
+	"time"
+)
+
+func TestProcmail(t *testing.T) {
+	dir, err := ioutil.TempDir("", "test-chasquid-courier")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	procmailBin = "tee"
+	procmailArgs = []string{dir + "/%user%"}
+
+	p := Procmail{}
+	err = p.Deliver("from@x", "to@y", []byte("data"))
+	if err != nil {
+		t.Fatalf("Deliver: %v", err)
+	}
+
+	data, err := ioutil.ReadFile(dir + "/to")
+	if err != nil || !bytes.Equal(data, []byte("data")) {
+		t.Errorf("Invalid data: %q - %v", string(data), err)
+	}
+}
+
+func TestProcmailTimeout(t *testing.T) {
+	procmailBin = "/bin/sleep"
+	procmailArgs = []string{"1"}
+	procmailTimeout = 100 * time.Millisecond
+
+	p := Procmail{}
+	err := p.Deliver("from", "to", []byte("data"))
+	if err != timeoutError {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	procmailTimeout = 1 * time.Second
+}
+
+func TestProcmailBadCommandLine(t *testing.T) {
+	p := Procmail{}
+
+	// Non-existent binary.
+	procmailBin = "thisdoesnotexist"
+	err := p.Deliver("from", "to", []byte("data"))
+	if err == nil {
+		t.Errorf("Unexpected success: %q %v", procmailBin, procmailArgs)
+	}
+
+	// Incorrect arguments.
+	procmailBin = "cat"
+	procmailArgs = []string{"--fail_unknown_option"}
+
+	err = p.Deliver("from", "to", []byte("data"))
+	if err == nil {
+		t.Errorf("Unexpected success: %q %v", procmailBin, procmailArgs)
+	}
+}