Attempt to address points brought up in #tor flamewar. In particular, moved

Attempt to address points brought up in #tor flamewar. In particular, moved
"Who will enable this option?" section towards the top of the proposal, to
attempt to get everyone on the same page right away as far as assumptions
go.

Also, added section on "Consideration of risks for node operators" where
the additional risk of should-be-3-but-actually-2 hop users pose to node
operators is discussed. Upon consideration of this, determined that two hop
users should be made to rotate guards with some frequency on the order of
days (basically, long enough to help scan the network for active adversary
guards, and then move on).

Please re-flame if you feel these or other issues have not been adequately
addressed.

svn:r10498