kernel: separate headers into kernel and user headers

all kernel headers have been moved from include/mango to include/kernel
and include definitions that are only relevant to kernel-space.

any definitions that are relevant to both kernel- and user-space
(i.e. type definitions, syscall IDs) have been moved to
include/mango within libmango.

arch/x86_64/thread.c

@@ -1,5 +1,5 @@
-#include <mango/machine/cpu.h>
-#include <mango/machine/thread.h>
+#include <kernel/machine/cpu.h>
+#include <kernel/machine/thread.h>
 
 /* this is the context information restored by ml_thread_switch.
  * since ml_thread_switch only jumps to kernel-mode, IRETQ isn't used,