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.

libc/string/memmove.c

@@ -1,5 +1,5 @@
 #include <stdint.h>
-#include <mango/libc/string.h>
+#include <kernel/libc/string.h>
 
 static void *memcpy_r(void *dest, const void *src, size_t sz)
 {