Input: document meanings of KEY_SCREEN and KEY_ZOOM

It is hard to say what KEY_SCREEN and KEY_ZOOM mean, but historically DVB
folks have used them to indicate switch to full screen mode. Later, they
converged on using KEY_ZOOM to switch into full screen mode and KEY)SCREEN
to control aspect ratio (see Documentation/media/uapi/rc/rc-tables.rst).

Let's commit to these uses, and define:

- KEY_FULL_SCREEN (and make KEY_ZOOM its alias)
- KEY_ASPECT_RATIO (and make KEY_SCREEN its alias)

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>