Feature: Add rc.pre-shutdown as new configuration file

[dataCobra]

Hello,

after installing void-linux on my laptop, I came pretty fast to the conclusion, that I need the option to execute some things before the services are stopped by runit.

For this I introducing rc.pre-shutdown as a configuration file for things that must be executed before the services are stopped.

I'm already using this on my daily driver laptop.

Since I don't yet know which other things need to be updated I hope you can help me with implementing this feature?

Regards, Ben