Use the XComposite extension to get the composite overlay window,
instead of just using the normal root window. This ensures that
composited windows are covered.

This time, we hide it behind a config option because it might break
things more than it fixes.

The code now tries the following options, in order:

• obtain monitors via RandR 1.5
• obtain outputs via RandR ≤ 1.4
• obtain screens via Xinerama

fixes #159
related to #101
related to #148
related to #149

This reverts commit 80d4452ec6.

fixes #130

Also apply two security measures for OpenBSD:
- use explicit_bzero(3)
- mlock(2) works for non-root users too

Use the XComposite extension to get the composite overlay window,
instead of just using the normal root window. This ensures that
composited windows are covered.

It has not been in use since commit 7eb43aeabd

Thanks to Ran Benita and Daniel Stone (the libxkbcommon authors) for
answering my questions and reviewing this code.

With this commit, input handling should be more correct with using less
code (in i3lock, that is).