0.3.0: absolute paths are rewritten

If an absolute path is specified for `ssh-tpm-keygen -f`, it becomes a relative path under `~/.ssh/`:

```
$ ssh-tpm-keygen -b 256 -f /tmp/ec
Generating a sealed public/private ecdsa key pair.
Enter file in which to save the key (/home/grawity/.ssh/tmp/ec):
```