dir-spec.txt: Update requirements for Guard/Fast flags.

   least two of the ports 80, 443, and 6667 and allows exits to at

   least one /8 address space.

   "Fast" -- A router is 'Fast' if it is active, and its bandwidth is

   either in the top 7/8ths for known active routers or at least some

   minimum (20KB/s until 0.2.3.7-alpha, and 100KB/s after that).

   "Fast" -- A router is 'Fast' if it is active, and its bandwidth is either in

   the top 7/8ths for known active routers or at least 100KB/s.

   "Guard" -- A router is a possible 'Guard' if its Weighted Fractional

   Uptime is at least the median for "familiar" active routers, and if

   its bandwidth is at least median or at least 250KB/s.

   "Guard" -- A router is a possible 'Guard' if its Weighted Fractional Uptime

   is at least the median for "familiar" active routers, and if its bandwidth

   is at least 2MB/s or is in the set of the 25% fastest relays.

        To calculate weighted fractional uptime, compute the fraction

        of time that the router is up in any given day, weighting so that