From here on, we’ll use clang-format to automatically format the source.

This has worked well in i3, so we are introducing it for i3lock.

See also:
http://article.gmane.org/gmane.linux.kernel/1268792

The C compiler will handle (void) as "no arguments" and () as "variadic
function" (equivalent to (...)) which might lead to subtle errors, such
as the one which was fixed with commit 0ea64ae4.