[Exim] Subkey queries and ldap

The manual only describes subkey queries for single-key lookups, but
the example uses nis+, a query lookup. Appearantly, subkeys don't work
for ldap queries, so I thought, I could use extract. That does not work
either, though, because ldap queries that return multiple results do not
separate them by " " but by ", " (line 356 in ldap.c). Changing that to
" " makes extract work properly. I guess this should be changed and the
manual should describe what the result of such queries is, as it does
with nis+.

Still, ldap subkey queries would be more readable than using extract.

I know, this topic has been discussed just a short while ago, but I
didn't have time to look at this issue closely until now.

Michael