Fix bunch of linter warnings

Some fixed warnings are not actual problems. I just want output
golangci-run to be smaller. Otherwise it is problematic to see
actual problems found by linters due to amount of noise.

testutils, ctl/maddyctl: Remove quite a lot of unused entities.
msgpipeline: Fix typo in log message.
msgpipeline: Add missing 'case 0' for 'reject' directive parsing
msgpipeline: Preallocate slice in parseChecksGroup
Check errors in some places in tests.
smtp_upstream: Move DATA command use into Commit
target/queue: Avoid copying sync.Mutex
    Not a problem since it is not used after copying, but I just want
    govet to shut up.

target/queue/queue_test.go

@@ -53,7 +53,9 @@ func newTestQueueDir(t *testing.T, target module.DeliveryTarget, dir string) *Qu
 		q.Log = testutils.Logger(t, "queue")
 	}
 
-	q.start(1)
+	if err := q.start(1); err != nil {
+		panic(err)
+	}
 
 	return q
 }