implement new 'zrepl status'

Primary goals:

- Scrollable output ( fixes #245 )
- Sending job signals from status view
- Filtering of output by filesystem

Implementation:

- original TUI framework: github.com/rivo/tview
- but: tview is quasi-unmaintained, didn't support some features
- => use fork https://gitlab.com/tslocum/cview
- however, don't buy into either too much to avoid lock-in

- instead: **port over the existing status UI drawing code
  and adjust it to produce strings instead of directly
  drawing into the termbox buffer**

Co-authored-by: Calistoc <calistoc@protonmail.com>
Co-authored-by: InsanePrawn <insane.prawny@gmail.com>

fixes #245
fixes #220

util/choices/choices_test.go

@@ -0,0 +1,50 @@
+package choices_test
+
+import (
+	"bytes"
+	"flag"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/zrepl/zrepl/util/choices"
+)
+
+func TestChoices(t *testing.T) {
+
+	var c choices.Choices
+
+	fs := flag.NewFlagSet("testset", flag.ContinueOnError)
+	c.Init("append", os.O_APPEND, "overwrite", os.O_TRUNC|os.O_CREATE)
+	fs.Var(&c, "mode", c.Usage())
+	var o bytes.Buffer
+	fs.SetOutput(&o)
+
+	fs.Usage()
+	usage := o.String()
+	o.Reset()
+
+	t.Logf("usage:\n%s", usage)
+	require.Contains(t, usage, "\"append\"")
+	require.Contains(t, usage, "\"overwrite\"")
+
+	err := fs.Parse([]string{"-mode", "append"})
+	require.NoError(t, err)
+	o.Reset()
+	require.Equal(t, os.O_APPEND, c.Value())
+
+	c.SetDefaultValue(nil)
+	err = fs.Parse([]string{})
+	require.NoError(t, err)
+	o.Reset()
+	require.Nil(t, c.Value())
+
+	// a little whitebox testing: this is allowed ATM, we don't check that the default value was specified as a choice in init
+	c.SetDefaultValue(os.O_RDWR)
+	err = fs.Parse([]string{})
+	require.NoError(t, err)
+	o.Reset()
+	require.Equal(t, os.O_RDWR, c.Value())
+
+}