Design change: Do keyboard polling internally without calling the kernal

Design change: Do keyboard polling internally without calling the kernal
in the system bank. For one this performs better (several %), second it
allows to handle the function keys in conformance with other platforms.
Without the custom keyboard routine, we would have to apply some more
magic to the function keys to make them work as with other cc65 targets.

git-svn-id: svn://svn.cc65.org/cc65/trunk@2811 b7a2c559-68d2-44c3-8de9-860c34a00d81