Prevent keystrokes from leaking through to the console when using evdev.

This uses the same method Weston and X use. Sadly, to be fully effective when
launching remotely, this needs root permissions.

README-raspberrypi.txt

@@ -148,5 +148,8 @@ this determining the CAPS LOCK behavior:
  Notes
 ================================================================================
 
-* Input events from the keyboard leak through to the console
+* When launching apps remotely (via SSH), SDL can prevent local keystrokes from
+  leaking into the console only if it has root privileges. Launching apps locally
+  does not suffer from this issue.
+