Allow vendors to tag version information to Tor
Currently we have no idea which vendors are responsible for which set of relays in the network, as we only have a version
tag in our descriptors.
It would be useful if a vendor could build tor with something like that: ./configure --vendor-name="debian" --vendor-version="0.3.5.2~deb1337"
and our version string would become something like: tor_version=0.3.5.2 vendor=debian vendor_version=0.3.5.2~deb1337
in some backwards compatible manner.