for_discovery option in add_an_entry_guard() is confusingly named

In #9946 (moved) I added a new argument "for_discovery" to add_an_entry_guard(). Nick prefers "provisional" or "probationary".

In parallel, I think we should probably rename the made_contact field in entry guard t, to be why we're remembering that we've made contact, rather than simply that we have.

And lastly, we should do something about the godawful number of int arguments that add_an_entry_guard() now takes.