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/pit.c

@@ -1,8 +1,8 @@
 #include <arch/irq.h>
 #include <arch/ports.h>
-#include <mango/clock.h>
-#include <mango/cpu.h>
-#include <mango/printk.h>
+#include <kernel/clock.h>
+#include <kernel/cpu.h>
+#include <kernel/printk.h>
 
 #define PIT_COUNTER0  0x40
 #define PIT_CMD       0x43