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.

ds/ringbuffer.c

@@ -1,5 +1,5 @@
-#include <mango/ringbuffer.h>
-#include <mango/sched.h>
+#include <kernel/ringbuffer.h>
+#include <kernel/sched.h>
 
 size_t ringbuffer_unread(struct ringbuffer *ring_buffer)
 {