[Fix] arc: emit ARC headers in a deterministic order

[Fix] arc: emit ARC headers in a deterministic order

lua_mime.modify_headers accepted an `order` list but it had no effect:
the headers passed through a string-keyed Lua table and were serialised
to the milter reply in arbitrary hash order. arc.lua relied on `order`
to lay out an ARC set, so the three ARC headers were emitted in a
non-deterministic order. Some validators (e.g. O365) reject ARC sets
that are not in the conventional ARC-Seal, ARC-Message-Signature,
ARC-Authentication-Results layout.

When `order` is given, emit one milter reply per header in that order
(set_milter_reply merges replies cumulatively, so a single-key reply
has no ambiguous iteration order) and apply the internal modify_header
calls in the same order.

Issue: #6045