debian: fix alias completion, rename ag and clean up README (#7698)

* Use double quotes to cache value of $apt_pref and $apt_upgr
* Clean up and fix syntax of command checks
* Clean up README and document $apt_pref/$apt_upgr overriding mechanism
* Rename `ag` alias (apt upgrade) to `au`
* Clean up README and fix syntax

Fixes #3686 
Fixes #4660 
Closes #5906

Co-authored-by: Noah Vesely <fowlslegs@riseup.net>