update systemd to persistence mixin

[h00die]

Pulls out systemd from the init persistence module and adds new persistence mixin. Part of #20374

Verification

• Start msfconsole
• exploit the box somehow (ssh_login for instance)
• use exploit/linux/persistence/init_systemd
• set SESSION <id>
• exploit
• Verify persistence is created, and you get a new session if apt is run
• Verify cleanup works
• Document is updated and correct

[msutovsky-r7]

Maybe using ::Faker::Hacker.say_something_smart here would be useful?

[h00die]

while this is easy to write defenses against, vs something more random... its relevant, and i worry that say_something_smart will give something too outlandish and not service related and make it obvious