unknown hosts: display a list of accepted hosts using the same key

When connecting to a host for which there's no known hostkey, check if the
selected key has been accepted for other hostnames.  This is useful when
connecting to a host with a dymamic IP address or multiple names.

An example where the local laptop is using DHCP and has 192.168.3.2 as its
current IP address:

```
% ssh 192.168.3.2
The authenticity of host '192.168.3.2 (192.168.3.2)' can't be established.
ECDSA key fingerprint is 9d:14:4f:ef:0c:0b:b4:1c:62:b0:89:af:e2:43:5c:4b.
You have previously accepted this key for the following hostnames:
        192.168.1.4
        localhost
Are you sure you want to continue connecting (yes/no)?
```