OS_Ust, Update small timer

Register parameters

RST 20H, DEFB $78

IN:  BC = new timer value
OUT:
     Fc = 0
     Fz = 0, old time value non-zero
     Fz = 1, old time value zero
     BC = old time value

Notes

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.