Update keyboard buffer on new input

[donno2048]

Makes both #130 and #132 irrelevant while still solving the issue

[donno2048]

As mentioned in #129 this also has the side effect of allowing a key combination to perform special operations to the emulator (like exiting) except in very rare edge cases

[dmsc]

This has a few problems:

• It will loose keys if you time multiple keys in sequence before emulator update, as queued_key is set to -1.
• Using poll is not that portable.

I will revamp keyboard handling by introducing a proper keyboard buffer, instead of "queued_key", the emulator should maintain a buffer of 16 entries in 40h:01Eh, this is what a real PC BIOS does and some programs depend on that.

At emulator_update, the keyboard should be read, keys added to the buffer and the pointer updated.

Currently, emu2 only updates the buffer pointer at 40h:01Ah.

Have Fun!