Consider replacing out.OutputIsRedirected by ctxutil.IsTerminal(ctx)

[AnomalRoil]

It seems we might have redundant handling of the "Are we in a terminal?" and "is output being redirected?"

ctxutil.IsTerminal relies on https://github.com/mattn/go-isatty in order to tell if we are in a terminal or not, which is great because it is very cross-platform, even supporting Plan9 and Windows.

I recently added in #2019 a method called OutputIsRedirected in our out package, which is meant to say if we are being redirected or not:

gopass/internal/out/print.go

Lines 30 to 34 in b490c57

	// OutputIsRedirected returns true if the current os.Stdout is a pipe instead of a terminal
	func OutputIsRedirected() bool {
	o, _ := os.Stdout.Stat()
	return (o.Mode() & os.ModeCharDevice) != os.ModeCharDevice
	}

It is not clear to me if this is equivalent to saying whether or not we are in a terminal, but it's pretty clear it probably isn't very cross-platform as is.

If "not being a tty" is including "being redirected", then we might want to change this new function to rely on go-isatty/ctxutil.IsTerminal(ctx) instead...

Originally posted by in #2024 (comment)

[AnomalRoil]

I just confirmed using the following example code from isatty:

package main

import (
	"fmt"
	"github.com/mattn/go-isatty"
	"os"
)

func main() {
	if isatty.IsTerminal(os.Stdout.Fd()) {
		fmt.Println("Is Terminal")
	} else if isatty.IsCygwinTerminal(os.Stdout.Fd()) {
		fmt.Println("Is Cygwin/MSYS2 Terminal")
	} else {
		fmt.Println("Is Not Terminal")
	}
}

That both piping and redirecting output returns "not a terminal", we can therefore replace the non-cross platform code for out.OutputIsRedirected with the cross platform ctxutil.IsTerminal function.

Sending a PR shortly.