Return the X atom name for VALUE as a string.
VALUE may be a number or a cons where the car is the upper 16 bits and
the cdr is the lower 16 bits of a 32 bit value.
Use the display for FRAME or the current frame if FRAME is not given or nil.
If the value is 0 or the atom is not known, return the empty string.
(fn VALUE &optional FRAME)