Having a random "cookie" here brings absolutely no security — it just
makes it more difficult to test.  Use predictable sequence numbers
instead.

This lets you get access to the underlying response object, in case you
want to get extra fields out of it.

These have diverged enough that splitting them out from each other
substantially improves readability.

We don't use this anywhere right now, but soon will.

The first component of the `host-key` protocol field is the host
name/IP, which is used to index the known_hosts key database. It's
confusing to call this `key_key`, as it's really not the key (nor
fingerprint) material.

In Fedora 41, systemd ships a new file
/etc/ssh/ssh_config.d/20-systemd-ssh-proxy.conf which is owned by
systemd_conf_t. Allow cockpit-ws to read that, otherwise the whole `ssh`
command fails with a SELinux denial as it can't read that config file.

This factorizes the current value and pins it down to a single query at
the time when clicking the "Login" button. That avoids a small race
condition where the user may change the field while the asynchronous
login process is running.