Previously, we'd just take all the nodes in EntryNodes, see which
ones were already in the guard list, and add the ones that weren't.
There were some problems there, though:
* We'd add _every_ entry in EntryNodes, and add them in the order
they appeared in the routerlist. This wasn't a problem
until we added the ability to give country-code or IP-range
entries in the EntryNodes set, but now that we did, it is.
(Fix: We now shuffle the entry nodes before adding them; only
add up to 10*NumEntryGuards)
* We didn't screen EntryNodes for the Guard flag. That's okay
if the user has specified two or three entry nodes manually,
but if they have listed a whole subcontinent, we should
restrict ourselves to the entries that are currently guards.
(Fix: separate out the new guard from the new non-guard nodes,
and add the Guards first.)
* We'd prepend new EntryNodes _before_ the already configured
EntryNodes. This could lead to churn.
(Fix: don't prepend these.)
This patch also pre-screens EntryNodes entries for
reachableaddresses/excludenodes, even though we check for that
later. This is important now, since we cap the number of entries
we'll add.
