Real hardware support, preemption

- Make OS work on real hardware (HP EliteBook Folio 1040 G2)
- Make kernel & userland preemtable
- Fix various deadlocks w.r.t. disabling ISRs
- Implement PS/2 keyboard driver
- Fix bootloader not recognizing ACPI v2.0+
- Add scrolling to VGA text driver
- Emulate FSGSBASE if not supported
- Fix erroneous OOM panics