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/strrchr.c

@@ -1,4 +1,4 @@
-#include <mango/libc/string.h>
+#include <kernel/libc/string.h>
 
 char *strrchr(const char *str, int c) {
     size_t len = strlen(str);