Add message directing users to security guide
Addresses #2142, which was blocking on updates to zcash/support/security.html. That page has now been added, so this message directing users to the site can be included.
It displays in the zcash-cli --help and --version message text, and on the zcashd metrics screen.
When generating a new service key, explicitly request a RSA1024 one.
The bitcoin P2P protocol has no support for the longer hidden service names
that will come with ed25519 keys, until it does, we depend on the old
hidden service type so make this explicit.
See #9214.
> This new feature is enabled by default if Bitcoin Core is listening, and a connection to Tor can be made. It can be configured with the -listenonion, -torcontrol and -torpassword settings. To show verbose debugging information, pass -debug=tor.
But it is correct to say that the feature is enabled *regardless* of whether a connection to Tor can be made.
I propose to clarify that so that users can eliminate these in their logs (when `listen=1` and no Tor).
And I think it's okay to clarify about the `listen` option, because on several occasions when I read this before I always assumed `listening` meant `server=1` which cost me a lot of time in troubleshooting.
```
2016-10-24 06:19:22.551029 tor: Error connecting to Tor control socket
2016-10-24 06:19:22.551700 tor: Not connected to Tor control port 127.0.0.1:9051, trying to reconnect
```
0.12.1
