whitespace and style fixes; from Tiago Cunha and one from me.
replace a few (x)malloc with (x)calloc to prevent potential integer
overflows; from Tiago Cunha
move duplicate kbd and mouse modifier parsing to a generic function;
from Tiago Cunha
clarify kbd vs mouse functions
move kbfunc and mousefunc closer together
add support for mouse based group {,r}cycle; from Rodrigo Mosconi.  not
bound by default.
since the root and event window are the same in the case of a button
event on the screen's root window, there's no need to pass down the
entire XButtonEvent event, at least to group_menu(), the only callback
which takes an argument at this point; instead use the already populated
screen.
type fixes
bring buttonpress and keypress event handlers slightly closer together