Add syntax check in KEYDOWN / KEYUP

[jupiterbjy]

From document:

KEYDOWN / KEYUP
Holds / Releases a key.

However, syntax checks does not trigger regardless of how many keys are supplied.

KEYDOWN 1 2 3 4
KEYUP 1 2 3 4

Following actually runs fine Which results only last key(in this case 4) to be pressed/unpressed.

It's always important to check documents but I think it might be even better to add strong warning like EMUK does with more than 1 key to not give a user a misperception.