nixos/sshguard: fix syslog ids, no more pid file, cleanups

1. Allow syslog identifiers with special characters
2. Do not write a pid file as we are running in foreground anyway
3. Clean up the module for readability

Without this, when deploying using nixops, restarting sshguard would make
nixops show an error about restarting the service although the service is
actually being restarted.