evdev: add a way for applications to tweak how devices are opened

In some cases, applications (or actually, wayland compositors)
don't have the required permissions to access evdev directly, but
can do so with an external helper like weston-launch.

Allow them to do so with a custom callback that replaces the regular
open() path.

https://bugzilla.gnome.org/show_bug.cgi?id=704269