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.

arch/x86_64/initcall.c

@@ -1,4 +1,4 @@
-#include <mango/init.h>
+#include <kernel/init.h>
 
 extern char __initcall0_start[];
 extern char __initcall1_start[];