This simplifies the implementation of remotes. The only required
interface is now `List` which is a simple one level directory list.
Optionally remotes may implement `ListR` if they have an efficient way
of doing a recursive list.
* Add options to Put, PutUnchecked and Update for all Fses
* Use these to create HashOption
* Implement this in local
* Pass the option in fs.Copy
This has the effect that we only calculate hashes we need to in the
local Fs which speeds up transfers significantly.
If the last FTP command caused an error, and if the error wasn't a
regular FTP error code, then we check the connection is working using
a NOOP call before returning it to the connection pool.
* fix root
* factor ftpConnection
* fix path munging
* fix recursive dir loops after update
* use fs.Trace and comment out debugs
* re-arrange and supplement docs
