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.

sched/runqueue.c

@@ -1,6 +1,6 @@
-#include <mango/sched.h>
-#include <mango/percpu.h>
-#include <mango/cpu.h>
+#include <kernel/sched.h>
+#include <kernel/percpu.h>
+#include <kernel/cpu.h>
 
 #define PRIO_MASK(p) (((uint32_t)1) << (p))
 #define FIRST_PRIO(m) (m > 0 ? (PRIO_MAX - __builtin_clz(m) - 1) : -1)