NodeName can include non-printable characters and thats causing troubles

I cannot think of a good use case for non-printable characters in a node name. It also causes problems:

* if it is a server participating in WAN, the non printable char will mess up the node name: https://github.com/hashicorp/consul/blob/65dd14daf304adf012cf1ba220ad0306ec952703/agent/consul/server_serf.go#L40
* the node name is not correctly shown on startup: https://github.com/hashicorp/consul/blob/65dd14daf304adf012cf1ba220ad0306ec952703/command/agent/agent.go#L243
* http api doesn't work with node name with non-printable char

```
$ consul agent -dev -node $(printf "node\x01")
```