ui: Simplify stdio wrapper

The StdioWrapper type is really just a pair of io.WriteClosers, so
remove it in favor of a function that returns two of those. Test
coverage increases because the removed code was not tested.

cmd/restic/termstatus.go

@@ -31,8 +31,7 @@ func setupTermstatus() (*termstatus.Terminal, func()) {
 
 	// use the termstatus for stdout/stderr
 	prevStdout, prevStderr := globalOptions.stdout, globalOptions.stderr
-	stdioWrapper := ui.NewStdioWrapper(term)
-	globalOptions.stdout, globalOptions.stderr = stdioWrapper.Stdout(), stdioWrapper.Stderr()
+	globalOptions.stdout, globalOptions.stderr = ui.WrapStdio(term)
 
 	return term, func() {
 		// shutdown termstatus