src: Add support for /dev/input devices

src: Add support for /dev/input devices

Plymouth currently gets keyboard input from the terminal. This isn't
ideal, since it means plymouth requires VTs to be enabled in the kernel.

Furthermore, most display servers use /dev/input and libxkbcommon for
keyboard handling these days.

This commit adds similar support to the plymouth core code. Subsequent
commits will add support to the render plugins.

Some contributions by n3rdopolis and Ray Strode.