open cursor after mysql_stmt_execute" + post-review fixes.
The bug was caused by wrong flags in stmt->server_status on the client
side: if there was no cursor, the server didn't send server_status
flags to the client, and the old flags were used to set up the
fetch function of a statement. Consequently, stmt_read_row_from_cursor was
used when there was no cursor. The fix fixes the server to always
send server flags to the client.