Add a fast-drop key

[juanmeleiro]

I really miss a fast-drop key (the down key in some implementations) for situations where I want to “tuck” a piece under another, and so have to wait for the falling one to drop all the way down.

I confess that I tried doing it myself, but got utterly confused by the code. Is that because I'm bad at C? Or because the original code was obfuscated? Who knows.

In particular, I leave these two questions:

1. How does the c < 0 branch of the main loop get executed? c is only updated via getchar(), which won't read an EOF from stdin, unless I am missing something…
2. How does the timer affect updates? It clearly does, because removing the signal handler halts updates. But it does nothing else other than just resetting the timer itself! Somehow, it's what appears to make the code go to the c < 0 branch.