Rely on onion_populate_cpath to check that we're only using
TAP for the rare hidden service cases.
Check and log if handshakes only support TAP when they should support
ntor.
If we know a node's version, and it can't do ntor, consider it not running.
If we have a node's descriptor, and it doesn't have a valid ntor key,
consider it not running.
Refactor these checks so they're consistent between authorities and clients.
Before, they checked for version 0.2.4.18-rc or later, but this
would not catch relays without version lines, or buggy or malicious
relays missing an ntor key.
teor (Tim Wilson-Brown)