Make NumEntryGuards consistent with #271 consensus params

Right now, NumEntryGuards tries to do some magic to set "guard-n-primary-guards-to-use" to the torrc value, and "guard-n-primary-guards" to twice that value.

This prevents us from testing our current favored Proposal 291 params (2 for each). So the torrc option could either be changed to set both of those values to the same, or we could make two torrc options.

We should also ensure that whatever we do, we have the ability to set the torrc such that we could get the same behavior as all existing clients would get, with those consensus params, if we wanted. This includes directory guards (which we currently believe will and should be the same as the two primary guards).

asn has a testing branch here: https://github.com/asn-d6/tor/tree/prop291_testing. This is not ready for merge, be could be adapted into a mergable, backportable thing.

He also has a useful guard log promotion patch here: https://github.com/asn-d6/tor/tree/guard_monitor