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.

ds/btree.c

@@ -57,7 +57,7 @@
    provide a comparator function.
 */ 
    
-#include <mango/btree.h>
+#include <kernel/btree.h>
 #include <stddef.h>
 
 #define MAX(a, b) ((a) > (b) ? (a) : (b))