Add buffer to PS/2 lib

[pulkomandy]

The PS/2 keyboard lib is built around an interrupt handler doing all the work and a blocking function to wait for a char.
There is no buffering at all, so as soon as we start receinving another char the first one is lost !

Fix this completely unsafe lib by adding a small buffer and turn it into non-blocking: allow polling for a char with immediate return if there is none in the buffer.

Same for Amiga keyboard lib.