Add option to give guard flag to relays without the CVE-2011-2768 fix

This way, all of the DA operators can upgrade immediately, without nuking
every client's set of entry guards as soon as a majority of them upgrade.

Until enough guards have upgraded, a majority of dirauths should set this
config option so that there are still enough guards in the network. After
a few days pass, all dirauths should use the default.