nixos/sshguard: create ipsets before starting, and clean up after stopping.

The fix for #62874 introduced a race condition on startup: the postStart
commands that configure the firewall run concurrently with sshguard's
creation of the ipsets that the rules depend on. Unfortunately iptables
fails hard when referencing an ipset that doesn't exist, so this causes
non-deterministic crashlooping until sshguard wins the race.

This change fixes that race condition by always creating the ipset and
reconfiguring the firewall before starting sshguard, so that the order
of operations is always deterministic.

This change also cleans up the ipsets on sshguard shutdown, so that
removing sshguard from a running system doesn't leave state behind.

Fixes #65985.