Implement proposal 318: Limit protovers to 0..63 [for 0.3.5 through 0.4.4]
In brief: we go through a lot of gymnastics to handle huge protover numbers, but after years of development we're not even close to 10 for any of our current versions. We also have a convenient workaround available in case we ever run out of protocols: if (for example) we someday need Link=64, we can just add Link2=0 or something.
This patch is a minimal patch to change tor's behavior; it doesn't take advantage of the new restrictions.
Implements #40133 and proposal 318.