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/local/read_device_other.go

@@ -8,6 +8,6 @@ import "os"
 
 // readDevice turns a valid os.FileInfo into a device number,
 // returning devUnset if it fails.
-func readDevice(fi os.FileInfo) uint64 {
+func readDevice(fi os.FileInfo, oneFileSystem bool) uint64 {
 	return devUnset
 }