OS_Ust, update small timer

RST 20H, DEFB $78
 

IN:  BC = new timer value

     Fc = 0
     Fz = 0, old time value non-zero
     Fz = 1, old time value zero
     BC = old time value

Internal operating system usage.
The timer latches at $0000. An initial value of $FFFF will not be decremented. The timer decrements once per centiseconds.

This code uses same timer as OS_Tin and OS_Dly.