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.

sched/core.c

@@ -1,10 +1,10 @@
-#include <mango/clock.h>
-#include <mango/cpu.h>
-#include <mango/machine/thread.h>
-#include <mango/object.h>
-#include <mango/printk.h>
-#include <mango/sched.h>
-#include <mango/vm-region.h>
+#include <kernel/clock.h>
+#include <kernel/cpu.h>
+#include <kernel/machine/thread.h>
+#include <kernel/object.h>
+#include <kernel/printk.h>
+#include <kernel/sched.h>
+#include <kernel/vm-region.h>
 
 extern kern_status_t setup_kernel_task(void);
 extern kern_status_t setup_idle_task(void);