obj: add header-offset field to object_type

this allows the object header to be placed anywhere within the larger
object structure.

the object system now also ensures that the object is zero-initialised
during allocation.

include/socks/object.h

@@ -48,6 +48,7 @@ struct object_type {
 	enum object_type_flags ob_flags;
 	char ob_name[32];
 	unsigned int ob_size;
+	unsigned int ob_header_offset;
 	struct vm_cache ob_cache;
 	struct queue_entry ob_list;
 	struct queue ob_attrib;