differentiating real and fake command events

[Psionic K]

I'm adding macro playback support to dslide.  My first pass is workable but
not literally fool proof.

The implementation sets `unread-command-events' using a recursive timer
function.  This was the first implementation I found that wouldn't eat the
events, as `sit-for' etc enjoy doing.  I would prefer a blocking
implementation to prevent any input from my playback to help save users
from screwing up.

If no blocking approach is feasible, I need ways to decide if the user
pressed a key.  I need to block this key if possible, but at least with
detection I can begin some recovery strategy.