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/drive/upload.go

@@ -58,7 +58,7 @@ func (f *Fs) Upload(in io.Reader, size int64, contentType string, fileID string,
 	if f.isTeamDrive {
 		params.Set("supportsTeamDrives", "true")
 	}
-	if *driveKeepRevisionForever {
+	if f.opt.KeepRevisionForever {
 		params.Set("keepRevisionForever", "true")
 	}
 	urls := "https://www.googleapis.com/upload/drive/v3/files"
@@ -197,11 +197,11 @@ func (rx *resumableUpload) Upload() (*drive.File, error) {
 	start := int64(0)
 	var StatusCode int
 	var err error
-	buf := make([]byte, int(chunkSize))
+	buf := make([]byte, int(rx.f.opt.ChunkSize))
 	for start < rx.ContentLength {
 		reqSize := rx.ContentLength - start
-		if reqSize >= int64(chunkSize) {
-			reqSize = int64(chunkSize)
+		if reqSize >= int64(rx.f.opt.ChunkSize) {
+			reqSize = int64(rx.f.opt.ChunkSize)
 		}
 		chunk := readers.NewRepeatableLimitReaderBuffer(rx.Media, buf, reqSize)