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 <[e-mail]>

include/uapi/linux/input-event-codes.h

···
 #define KEY_TITLE		0x171
 #define KEY_SUBTITLE		0x172
 #define KEY_ANGLE		0x173
-#define KEY_ZOOM		0x174
+#define KEY_FULL_SCREEN		0x174	/* AC View Toggle */
+#define KEY_ZOOM		KEY_FULL_SCREEN
 #define KEY_MODE		0x175
 #define KEY_KEYBOARD		0x176
-#define KEY_SCREEN		0x177
+#define KEY_ASPECT_RATIO	0x177	/* HUTRR37: Aspect */
+#define KEY_SCREEN		KEY_ASPECT_RATIO
 #define KEY_PC			0x178	/* Media Select Computer */
 #define KEY_TV			0x179	/* Media Select TV */
 #define KEY_TV2			0x17a	/* Media Select Cable */