Closes the Doors CS GUI system and removes the gui6 appvar from memory. No ill effects can be accidentally incurred by calling this routine multiple times, although the return value of the c register is used to indicate whether the stack had already been closed. This routine is the counterpart of the OpenGUIStack function. Warning: this will delete any information currently in the GUI stack. Copy any important data into files or saferam before closing the stack.
c = 0 if the stack was open, now close; 1 if the stack was already closed.
hl = size of stack before close if c=0.
de = where stack was if c=0.
(Previous two register values useful for ptr updating)
a, bc, de, hl, Op1