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/nettrace/trace.go

@@ -233,7 +233,7 @@ func (tr *trace) Printf(format string, a ...interface{}) {
 func (tr *trace) Errorf(format string, a ...interface{}) error {
 	tr.SetError()
 	err := fmt.Errorf(format, a...)
-	tr.Printf(err.Error())
+	tr.Printf("%s", err.Error())
 	return err
 }