OS_Ploz, Poll for running OZ

IN:
     A = slot number (0 to 3)

OUT, if call successfull:
     Fz = 0, OZ is running in slot
     Fz = 1, OZ is not running in slot.

OUT, if call failed:
     Fc = 1
     A = RC_ONF, no card found

Registers changed after return:
     A.BCDEHL/IXIY same
     .F....../.... different 

OZ can only run in slot 0 or 1. Using this call for slots 2 and 3 will always return Fz = 1. This call is used internally by OZ in the Flash card management sub-system, when it is necessary to turn off the screen during Flash hardware operations on the same card as a running OZ. Applications may benefit from this system call too.