login – device name prompt shows wrong error message

`keybase login` seems to be rejecting device names with dots, but doesn't want to admit that:

```
Enter a public name for this device: rain.nullroute.eu.org
Enter a public name for this device (between 3 and 64 characters long): rain.nullroute.eu.org
Enter a public name for this device (between 3 and 64 characters long): rain
```

(I counted just in case, it's 21 characters.)

Arch Linux aur/keybase-release 4428.3e4cfb2-1

Start with the `keybase login` device-name prompt and reproduce the issue using `rain.nullroute.eu.org` and `rain`. Trace the validation that rejects the dotted name and the message it displays; done means the error accurately describes the rejected input or accepts it if dots are valid.

Written by the indexing model from the issue text.