[exim] Double use of message acl modifier dissatisfactory

I'm a little bit disappointed with how the only way to add a header line
in an ACL is with the warn verb. I you want to accept with an added
header under certain circumstances, but pass control on to other
statements, which could accept or reject, perhaps adding other header
line, then you either have to use sub-ACLs or repeat all the conditions.

I'd like to introduce a new modifier, e.g. "header", and deprecate the
use of message to add header lines. This doesn't make the warn verb
meaningless.

Cheers
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Magnus Holmgren