Add correct sleep which works now because interrupts are re-enabled

using sti in syscalls, remove a bunch of old imports and code, move
keyboard scancode handling to keyboard.rs, add a new KeyboardEvent based
layout where press/release and unicode is handled, add a kbd_read
syscall which writes kbd events to a userspace buffer, add a usercopy
file which provides safe copying functions to userspace

user/apps/helloworld/helloworld.c

@@ -1,6 +1,6 @@
-#include "../src/stdio.h"
+#include "../../libxunil/include/stdio.h"
 
-void _start() {
-   write(0, "Hello, World!", 13);
+int main(int argc, char **argv) {
+   printf("%s\n", "Hello, world!");
    exit(0);
 }