Added support for SFTP servers that do not allow SSH commands (e.g. Paramiko)

[kenkendk]

The SFTP server Paramiko (and other strictly SFTP configured servers) do not permit side-channel SSH commands.

The current implementation requests these, but does not supply a timeout or fault handler for these.

This patch provides a 10 second timeout, and marks SSH commands as non-supported, for future requests. If the SSH commands are not supported, a FXP_STAT request is made on "." for extracting the UID and GID. The "df" command simply returns the default values, if statvfs is not supported.