user callbacks for handling interrupt signal while waiting on receive

[parkrish]

In nc_session_receive, on interrupt we break the poll and then continue.
However, the interruption from the client application would have come as a result of user choosing to break the receive may be due to a long wait. In this case it may not be good to continue.

It will be good if an API is present to control a variable by user application, which can be used here and choose to continue or not depending on the variable value.

} else if (((status == -1) && (errno == EINTR))
#ifndef DISABLE_LIBSSH
|| (status == SSH_AGAIN)
#endif
) {
/* poll was interrupted */
continue;