Implement new backend config system

This unifies the 3 methods of reading config

  * command line
  * environment variable
  * config file

And allows them all to be configured in all places.  This is done by
making the []fs.Option in the backend registration be the master
source of what the backend options are.

The backend changes are:

  * Use the new configmap.Mapper parameter
  * Use configstruct to parse it into an Options struct
  * Add all config to []fs.Option including defaults and help
  * Remove all uses of pflag
  * Remove all uses of config.FileGet

fs/fs_test.go

@@ -4,6 +4,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/spf13/pflag"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -52,3 +53,20 @@ func TestFeaturesDisableList(t *testing.T) {
 	assert.False(t, ft.CaseInsensitive)
 	assert.False(t, ft.DuplicateFiles)
 }
+
+// Check it satisfies the interface
+var _ pflag.Value = (*Option)(nil)
+
+func TestOption(t *testing.T) {
+	d := &Option{
+		Name:  "potato",
+		Value: SizeSuffix(17 << 20),
+	}
+	assert.Equal(t, "17M", d.String())
+	assert.Equal(t, "SizeSuffix", d.Type())
+	err := d.Set("18M")
+	assert.NoError(t, err)
+	assert.Equal(t, SizeSuffix(18<<20), d.Value)
+	err = d.Set("sdfsdf")
+	assert.Error(t, err)
+}