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/cache.c

@@ -1,8 +1,8 @@
-#include <mango/queue.h>
-#include <mango/compiler.h>
-#include <mango/panic.h>
-#include <mango/vm.h>
-#include <mango/util.h>
+#include <kernel/queue.h>
+#include <kernel/compiler.h>
+#include <kernel/panic.h>
+#include <kernel/vm.h>
+#include <kernel/util.h>
 #include <stddef.h>
 
 #define FREELIST_END ((unsigned int)-1)