Fix non-constant format string in calls to Printf-like functions

In a few places, we call Printf-like functions, but for the format we
use either non-format messages (which is not tidy, but okay), or
variable messages (which can be problematic if they contain %-format
directives).

The patch fixes the calls by either moving to Print-like functions, or
using `Printf("%s", message)` instead.

These were found by a combination of `go vet` (which complains about
"non-constant format string in call"), and manual inspection.

internal/smtpsrv/server_test.go

@@ -408,7 +408,7 @@ func TestTooMuchData(t *testing.T) {
 
 func simpleCmd(t *testing.T, c *smtp.Client, cmd string, expected int) string {
 	t.Helper()
-	if err := c.Text.PrintfLine(cmd); err != nil {
+	if err := c.Text.PrintfLine("%s", cmd); err != nil {
 		t.Fatalf("Failed to write %s: %v", cmd, err)
 	}