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.

util/data_size.c

@@ -1,5 +1,5 @@
 #include <stddef.h>
-#include <mango/libc/stdio.h>
+#include <kernel/libc/stdio.h>
 
 struct magnitude {
     size_t threshold;

util/endian.c

@@ -1,4 +1,4 @@
-#include <mango/machine/cpu.h>
+#include <kernel/machine/cpu.h>
 #include <stdint.h>
 
 static uint16_t swap_uint16( uint16_t val )

util/random.c

@@ -1,4 +1,4 @@
-#include <mango/util.h>
+#include <kernel/util.h>
 
 static unsigned int random_seed = 53455346;