Embedded lock in history struct and fixed default --identity key location

> is 'ssh_host_rsa_key' a standard name for a key?

Yes, at least on Debian systems, the following keys are automatically generated when you install SSH (not sure if they are only created when you install `openssh-server`, or if they are created with `openssh-client` as well):

```
-rw------- 1 root root    Aug 30 01:43 ssh_host_dsa_key
-rw-r--r-- 1 root root    Aug 30 01:43 ssh_host_dsa_key.pub
-rw------- 1 root root    Aug 30 01:43 ssh_host_ecdsa_key
-rw-r--r-- 1 root root    Aug 30 01:43 ssh_host_ecdsa_key.pub
-rw------- 1 root root    Aug 30 01:43 ssh_host_ed25519_key
-rw-r--r-- 1 root root     Aug 30 01:43 ssh_host_ed25519_key.pub
-rw------- 1 root root   Aug 30 01:43 ssh_host_rsa_key
-rw-r--r-- 1 root root    Aug 30 01:43 ssh_host_rsa_key.pub
```