@brief Read current values of a set of GPIO lines. @param bulk Set of GPIO lines to reserve. @param values An array big enough to hold line_bulk->num_lines values. @return 0 is the operation succeeds. In case of an error this routine returns -1 and sets the last error number.
If succeeds, this routine fills the values array with a set of values in the same order, the lines are added to line_bulk. If the lines were not previously requested together, the behavior is undefined.