Implementation of sftp_server_free() as counterpart to sftp_server_new().
The sftp_free() function calls ssh_channel_free(), not sftp_server_free().
I use the libssh as SFTP server. So I have to use the sftp_server_new() function, which does not create a channel in the opposite to sftp_new().
Also, it is less confusing if the new() function is named like the free() function.