Use the same function to join the root paths for the wrapping remotes
alias, cache and crypt.
The new function fspath.JoinRootPath is equivalent to path.Join, but if
the first non empty element starts with "//", this is preserved to allow
Windows network path to be used in these remotes.
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
