make file sharing protocol agnostic (support nfs, sshfs, etc..)

[Antoine Martin]

The data structures should be ok (generic enough).

What we need is to be able to hook any number of filesystem backends at both ends (and configure/detect which ones are available and enabled).
The code also needs to me moved out of client_channel.add_mount_point - this was done for expediency, but definitely not the right place for it!

This will also make it fairly easy to support client-mounts-server mode (not going to work for win32 mounting smb via tunnels - unless they have fixed it in newer versions of windoze)