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/percpu.c

@@ -1,6 +1,6 @@
-#include <mango/percpu.h>
-#include <mango/cpu.h>
-#include <mango/vm.h>
+#include <kernel/percpu.h>
+#include <kernel/cpu.h>
+#include <kernel/vm.h>
 #include <stdint.h>
 #include <stddef.h>