[BIOS] system: manual sleep handling

[asiekierka]

This covers the following API calls:

• INT 17h/AH=04h: sys_sleep
• INT 17h/AH=07h: sys_set_awake_key
• INT 17h/AH=08h: sys_get_awake_key
• INT 17h/AH=09h: sys_set_keepalive_int

[asiekierka]

ed231fc

Needs testing, but appears reasonable enough on first glance.