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/gdt.c

@@ -1,7 +1,7 @@
 #include <arch/gdt.h>
 #include <arch/tss.h>
-#include <mango/libc/string.h>
-#include <mango/types.h>
+#include <kernel/libc/string.h>
+#include <kernel/types.h>
 #include <stddef.h>
 
 static void init_entry(struct gdt_entry *entry, int access, int flags)