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