Delay ACL header edits until transport time.

Don't use the `add_header' ACL control any more.  Instead, just
accumulate the desired header additions and removals in variables, and
apply them at transport time.

This way, the headers we see in the message are the unmodified ones, as
the message was originally given to us.  We can therefore apply header
/removals/ (which aren't allowed in ACLs, so have to be delayed to
routing/transport time) coherently, without the risk of clobbering
the headers we've added ourselves.