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.go

@@ -0,0 +1,93 @@
+package courier
+
+import (
+	"bytes"
+	"fmt"
+	"os/exec"
+	"strings"
+	"time"
+
+	"blitiri.com.ar/go/chasquid/internal/trace"
+)
+
+var (
+	// Location of the procmail binary, and arguments to use.
+	// The string "%user%" will be replaced with the local user.
+	procmailBin  = "procmail"
+	procmailArgs = []string{"-d", "%user%"}
+
+	// Give procmail 1m to deliver mail.
+	procmailTimeout = 1 * time.Minute
+)
+
+var (
+	timeoutError = fmt.Errorf("Operation timed out")
+)
+
+// Procmail delivers local mail via procmail.
+type Procmail struct {
+}
+
+func (p *Procmail) Deliver(from string, to string, data []byte) error {
+	tr := trace.New("Procmail", "Deliver")
+	defer tr.Finish()
+
+	// Get the user, and sanitize to be extra paranoid.
+	user := sanitizeForProcmail(userOf(to))
+	tr.LazyPrintf("%s  ->  %s (%s)", from, user, to)
+
+	// Prepare the command, replacing the necessary arguments.
+	args := []string{}
+	for _, a := range procmailArgs {
+		args = append(args, strings.Replace(a, "%user%", user, -1))
+	}
+	cmd := exec.Command(procmailBin, args...)
+
+	cmdStdin, err := cmd.StdinPipe()
+	if err != nil {
+		return tr.Errorf("StdinPipe: %v", err)
+	}
+
+	output := &bytes.Buffer{}
+	cmd.Stdout = output
+	cmd.Stderr = output
+
+	err = cmd.Start()
+	if err != nil {
+		return tr.Errorf("Error starting procmail: %v", err)
+	}
+
+	_, err = bytes.NewBuffer(data).WriteTo(cmdStdin)
+	if err != nil {
+		return tr.Errorf("Error sending data to procmail: %v", err)
+	}
+
+	cmdStdin.Close()
+
+	timer := time.AfterFunc(procmailTimeout, func() {
+		cmd.Process.Kill()
+	})
+	err = cmd.Wait()
+	timedOut := !timer.Stop()
+
+	if timedOut {
+		return tr.Error(timeoutError)
+	}
+	if err != nil {
+		return tr.Errorf("Procmail failed: %v - %q", err, output.String())
+	}
+	return nil
+}
+
+// sanitizeForProcmail cleans the string, leaving only [a-zA-Z-.].
+func sanitizeForProcmail(s string) string {
+	valid := func(r rune) rune {
+		switch {
+		case r >= 'A' && r <= 'Z', r >= 'a' && r <= 'z', r == '-', r == '.':
+			return r
+		default:
+			return rune(-1)
+		}
+	}
+	return strings.Map(valid, s)
+}