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.

vm/flat.c

@@ -15,9 +15,9 @@
    memory outstripts free memory), the sparse memory model may
    be a better choice.
  */
-#include <mango/vm.h>
-#include <mango/memblock.h>
-#include <mango/printk.h>
+#include <kernel/vm.h>
+#include <kernel/memblock.h>
+#include <kernel/printk.h>
 
 /* array of pages, one for each physical page frame present in RAM */
 static struct vm_page *page_array = NULL;