[Exim] New exim code having DB writing problems

I've got callout verification caching code writting and (nearly)
working. As it turned out, it was quite easy to do, owing to how well
organized and understandable the Exim sources are. (I'm working with
Exim 4.10, of course).

I've only got one problem left... my code will successfully create the
callout cache database (same place as all the other hint databases), and
will add a single record the first time it gets used.

After that, it attempts to write additional records when needed, but
they never show up in the database file. dbfn_write is returning a
result of 13, but I can't figure what that's supposed to mean.

If you're out there Philip, give me a pointer as to what you think might
be going on. When this issue is resolved, I'll post a patch to the list
for others to try out.

The feature list is:
- all callout verification results in records being added to the hint
database, unless the callout defers
- records are marked as positive or negative
- two new config options, "callout_positive_expire" (default 24 hours)
and "callout_negative_expire" (default 2 hours) control how long records
are kept in the database