Fix IsProcessBackground on Linux with stdin redirection

The previous implementation assumed that stdin was a terminal.
It now checks the terminal's fd.

internal/ui/termstatus/background_linux_test.go

@@ -0,0 +1,19 @@
+package termstatus
+
+import (
+	"os"
+	"testing"
+
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestIsProcessBackground(t *testing.T) {
+	tty, err := os.Open("/dev/tty")
+	if err != nil {
+		t.Skipf("can't open terminal: %v", err)
+	}
+	defer tty.Close()
+
+	_, err = isProcessBackground(tty.Fd())
+	rtest.OK(t, err)
+}