all the way up to the user interface. All code modules bug sftp.c have
been completed.
Functions that return an "int", or similar return LIBSSH2CHANNEL_EAGAIN to
indicate some part of the call would block, in non-blocking mode.
Functions that return a structure, like "LIBSSH2_CHANNEL *", return NULL
and set the libssh2 error. The error can be obtained with either
libssh2_session_last_error() or libssh2_session_last_errno(). Either of
these will return the error code of LIBSSH2_ERROR_EAGAIN if the
call would block, in non-blocking mode.
The current state of a function and some variable are keep in the
structures so that on the next call the operation that would block can
be retried again with the same data.
