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.

syscall/dispatch.c

@@ -1,6 +1,6 @@
-#include <mango/machine/cpu.h>
-#include <mango/printk.h>
-#include <mango/syscall.h>
+#include <kernel/machine/cpu.h>
+#include <kernel/printk.h>
+#include <kernel/syscall.h>
 
 #define SYSCALL_TABLE_ENTRY(id, p) [SYS_##id] = (virt_addr_t)(sys_##p)