- Use require.Regexp to expect ssh handshake error
- Use require.Eventually to refactor verifyStatus

Due to the way sshd works, gitlab-shell could be called with a single
string in the form:

```
/path/to/gitlab-shell -c key-id
```

However, due to the tightening of the regular expressions in fcff692b
this string no longer matches, so logins would fail with:

```
Failed to get username: who='' is invalid
```

This can be reproduced by changing the user's shell to point to
gitlab-shell. For example:

```
usermod git -s /opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell
```

While setting gitlab-shell as the user's shell isn't officially
supported, gitlab-shell still should be able to cope with the key being
specified as the last argument. We now split the argument list and use
the last value.

Relates to https://gitlab.com/gitlab-org/gitlab-shell/-/issues/530

Improve logging for non-git commands

See merge request gitlab-org/gitlab-shell!538

Several of our commands only touch the internal API, and go nowhere
near Gitaly. Improve logging for each of these in a single MR. In
general, we want to be able to tell what happened in the execution of
each command, and to track failures down to a specific line of code.

Changelog: added

This will help to prevent misconfigurations.

Changelog: fixed

Use reflection to log the command we are about to execute, both in
gitlab-shell and gitlab-sshd. Include the environment, which has all
the context we need to understand what the command is expected to do.

Changelog: added

Currently we get this in some log messages:

logrus_error="can not add field \"channel_type\""

This is because we're trying to add a function, rather than the result
of the function call (a string) to the `log.Fields`. Whoops!

Changelog: fixed