userdb: rename userdb lookup flags a bit

Let's use "exclude" for flags that really exclude records from our
lookup. Let's use "avoid" referring to concepts that when flag is set
we'll not use but we have a fallback path for that should yield the same
result. Let' use "suppress" for suppressing partial info, even if we
return the record otherwise.

So far we used "avoid" for all these cases, which was confusing.

Whiel we are at it, let's reassign the bits a bit, leaving some space
for bits follow-up commits are going to add.