Userland: Add LibSystem and funnel all syscalls through it

This achieves two things:

- Programs can now intentionally perform arbitrary syscalls by calling
  syscall(). This allows us to work on things like syscall fuzzing.

- It restricts the ability of userspace to make syscalls to a single
  4KB page of code. In order to call the kernel directly, an attacker
  must now locate this page and call through it.

Userland/Libraries/LibKeyboard/CharacterMap.cpp

@@ -26,7 +26,6 @@
 
 #include "CharacterMap.h"
 #include <AK/StringBuilder.h>
-#include <Kernel/API/Syscall.h>
 #include <LibKeyboard/CharacterMapFile.h>
 
 #ifndef KERNEL