[exim] HELO and SPF - an intermediate solution

Here's a first go at sorting out the SPF / HELO issue - what this does
is allow the spf condition to be used in the acl_smtp_helo acl, and if
it is used there, magically checks the sender_helo_name instead of
anything else.

Behaviour in other ACLs is unchanged.

Patch is against 4.66.

Depending on feedback, I'll take this forward to the more generalised
implementation I suggested in bug 464, and fix the outstanding bug
with null senders (new bug 467)

Peter
--
Peter Bowyer
Email: peter@???