Accept shorter version numbers
I was thinking about removing the leading 0 from our version numbers, and realized that we can't[*] do that yet: tor_version_parse assumes that if you have a three-number version, you may not have a tag.
Let's allow two-number versions and three-number versions to have tags. And let's look into other ways to fix tor_version_parse.
[*] There are other ways to remove our leading 0. We could use four-number for alphas, and three-number versions for stable versions, as in "22.214.171.124-alpha, 126.96.36.199-alpha, 188.8.131.52-alpha, 184.108.40.206-rc, 2.7.5". Or we could have Tor report its vesion as 2.7.1-alpha to the UI, but advertise it as 0.2.7.1-alpha to other tors to work around this bug.