Initialises keyboard interrupts.
The following tasks are carried out by this routine:
- The keyboard buffer is flushed of any characters.
- The keyboard interrupt rate KBW_TDEL is set to $B3DD.
- The initial delay and key repeat rate KBB_DLAY, KBB_REPT are set
to 14 and 0 respectively.
- The keyboard status flags, in KBB_STAT are set to 0, ie KY_SHFT,
KY_CPNM, KY_CAPS and KY_NUMB are cleared.
- The length of the key click, KBB_CLIK, is set to 1.
- The interrupts are enabled: FREE RUNNING COUNTER is set to 0, TIMER
1 OUTPUT COMPARE REGISTER 1 is set to $B3DD, bit 3 of TIMER 1 CONTROL
STATUS REGISTER 1 is set, to enable the interrupt, and the I mask bit
of the condition code register is cleared.