Consider and check for availability of windows default sftp-server

[anjannath]

Currently AFAIK sshocker is only looking for the sftp-server provided by cygwin, which means it won't work if we don't have cygwin installed, since windows also provides an openSSH server, maybe that can be used, which would require users to just enable it from the optional features, once enabled the binary is present at: C:/Windows/System32/OpenSSH/sftp-server.exe