GitLab

* Switch order of "apt" and "aptitude" for detection

"apt" is installed by default at Debian (maybe Ubuntu too), while "aptitude" does not seem to be installed by default.
For that, it may be better for most of the users to prefer "aptitude" if installed.

* plugins/debian/README: Change of order mirroded into documentation

* 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>