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/LibCore/File.cpp

@@ -25,7 +25,6 @@
  */
 
 #ifdef __serenity__
-#    include <Kernel/API/Syscall.h>
 #    include <serenity.h>
 #endif
 #include <AK/ScopeGuard.h>