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

backend/alias/alias.go

@@ -7,7 +7,8 @@ import (
 	"strings"
 
 	"github.com/ncw/rclone/fs"
-	"github.com/ncw/rclone/fs/config"
+	"github.com/ncw/rclone/fs/config/configmap"
+	"github.com/ncw/rclone/fs/config/configstruct"
 )
 
 // Register with Fs
@@ -17,29 +18,42 @@ func init() {
 		Description: "Alias for a existing remote",
 		NewFs:       NewFs,
 		Options: []fs.Option{{
-			Name: "remote",
-			Help: "Remote or path to alias.\nCan be \"myremote:path/to/dir\", \"myremote:bucket\", \"myremote:\" or \"/local/path\".",
+			Name:     "remote",
+			Help:     "Remote or path to alias.\nCan be \"myremote:path/to/dir\", \"myremote:bucket\", \"myremote:\" or \"/local/path\".",
+			Required: true,
 		}},
 	}
 	fs.Register(fsi)
 }
 
+// Options defines the configuration for this backend
+type Options struct {
+	Remote string `config:"remote"`
+}
+
 // NewFs contstructs an Fs from the path.
 //
 // The returned Fs is the actual Fs, referenced by remote in the config
-func NewFs(name, root string) (fs.Fs, error) {
-	remote := config.FileGet(name, "remote")
-	if remote == "" {
-		return nil, errors.New("alias can't point to an empty remote - check the value of the remote setting")
-	}
-	if strings.HasPrefix(remote, name+":") {
-		return nil, errors.New("can't point alias remote at itself - check the value of the remote setting")
-	}
-	fsInfo, configName, fsPath, err := fs.ParseRemote(remote)
+func NewFs(name, root string, m configmap.Mapper) (fs.Fs, error) {
+	// Parse config into Options struct
+	opt := new(Options)
+	err := configstruct.Set(m, opt)
 	if err != nil {
 		return nil, err
 	}
-
-	root = filepath.ToSlash(root)
-	return fsInfo.NewFs(configName, path.Join(fsPath, root))
+	if opt.Remote == "" {
+		return nil, errors.New("alias can't point to an empty remote - check the value of the remote setting")
+	}
+	if strings.HasPrefix(opt.Remote, name+":") {
+		return nil, errors.New("can't point alias remote at itself - check the value of the remote setting")
+	}
+	_, configName, fsPath, err := fs.ParseRemote(opt.Remote)
+	if err != nil {
+		return nil, err
+	}
+	root = path.Join(fsPath, filepath.ToSlash(root))
+	if configName == "local" {
+		return fs.NewFs(root)
+	}
+	return fs.NewFs(configName + ":" + root)
 }