[Exim] Exim SRS implementation update.

I've updated the SRS implementation at http://www.infradead.org/rpr.html
so that it can handle rewriting for sites checking Caller-ID and those
which reject mail which originates from themselves.

For the CID bit, you have to include '_' in characters you're willing to
look up in domain names:

    dns_check_names_pattern = (?i)^(?>(?(1)\.|())[_a-z0-9\xc0-\xff]\
    (?>[-_a-z0-9\x80-\xff]*[_a-z0-9\x80-\xbf])?)+$

I've also fixed a bug which made it not-case-preserving when rewriting
addresses.

--
dwmw2