[exim] Portability check: writev()

This is for folks running Exim on platforms which are no longer
predominant.

Is anyone actively using Exim on a platform without a POSIX writev()
interface? If so, are you also using SpamAssassin?

Anything Linux, *BSD, Solaris, etc, are all fine, no need to reply to
let me know how awesome your distribution is; I'm trying to avoid
issues for folks using variations of Unix that are no longer receiving
feature updates for standards compatibility for something standardised
in 2001, and fairly widespread before then.

At present, the only use of writev() is in a couple of optional features
(Cyrus SASL integration; content scanning) and we want to know how safe
it is to use it more widely without worrying about feature-disabling
knobs.

Thanks,
-Phil