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.

kernel/object.c

@@ -1,6 +1,6 @@
-#include <mango/locks.h>
-#include <mango/object.h>
-#include <mango/queue.h>
+#include <kernel/locks.h>
+#include <kernel/object.h>
+#include <kernel/queue.h>
 
 #define HAS_OP(obj, opname) ((obj)->ob_type->ob_ops.opname)