Use context.WithTimeout instead of context.WithDeadline

There are a few context.WithDeadline calls that can be simplified by
using context.WithTimeout.

At the time they were added, WithTimeout was too new so we didn't want
to depend on it. But now that the minimum Go version has been raised to
1.9, we can simplify the calls.

This patch does that simplification, which is purely mechanical, and
does not change the logic itself.

internal/courier/procmail.go

@@ -58,8 +58,7 @@ func (p *Procmail) Deliver(from string, to string, data []byte) (error, bool) {
 	}
 	tr.Debugf("%s %q", p.Binary, args)
 
-	ctx, cancel := context.WithDeadline(context.Background(),
-		time.Now().Add(p.Timeout))
+	ctx, cancel := context.WithTimeout(context.Background(), p.Timeout)
 	defer cancel()
 	cmd := exec.CommandContext(ctx, p.Binary, args...)
 	cmd.Stdin = bytes.NewReader(data)

internal/queue/queue.go

@@ -381,8 +381,7 @@ func (item *Item) deliver(q *Queue, rcpt *Recipient) (err error, permanent bool)
 		if len(c) == 0 {
 			return fmt.Errorf("empty pipe"), true
 		}
-		ctx, cancel := context.WithDeadline(context.Background(),
-			time.Now().Add(30*time.Second))
+		ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
 		defer cancel()
 		cmd := exec.CommandContext(ctx, c[0], c[1:]...)
 		cmd.Stdin = bytes.NewReader(item.Data)

internal/smtpsrv/conn.go

@@ -743,8 +743,7 @@ func (c *Conn) runPostDataHook(data []byte) ([]byte, bool, error) {
 	defer tr.Finish()
 	tr.Debugf("running")
 
-	ctx, cancel := context.WithDeadline(context.Background(),
-		time.Now().Add(1*time.Minute))
+	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Minute)
 	defer cancel()
 	cmd := exec.CommandContext(ctx, c.postDataHook)
 	cmd.Stdin = bytes.NewReader(data)