Add eventfd support when accepting a new session using libssh channels

[plummer86]

Found that the nc_session transport_socket was not being set for sessions that are accepted using nc_session_accept_libssh_channel().

Needed so that the file descriptor can be returned from nc_session_get_eventfd().

[michalvasko]

I just changed it to fd_input and fd_output, we do not set transport_socket in server sessions.

Regards, Michal