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There appears to be a definition issue for some functions/aliases which
result in the following errors when sourcing .zshrc:

```
/home/username/.oh-my-zsh/plugins/debian/debian.plugin.zsh:75: defining function based on alias `abd'
/home/username/.oh-my-zsh/plugins/debian/debian.plugin.zsh:75: parse error near `()'
```

Fixes #7986

* 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>

* use https everywhere

* use https links on the files that are left

Also, removed some broken links and updated redirections.

Signed-off-by: Sascha Rudolph <sascharenerudolph@gmail.com>

ag is a famous command just like grep.

Changing from `zcat` to `zgrep` because some `zcat` implementations do not work
if the file is not compressed.

Improving with "safe-upgrade" for aptitude and "upgrade" for apt-get

The debian plugin calls which to determine whether aptitude & sudo are
installed.  However, if these are not installed then the error output
from which will be output.  Since this isn't really an error condition,
this commit updates the which calls to pipe stderr to stdout, thus
hiding the error messages.

Just an idea: change it to ap

Make the $apt_pref variable evaluatet at alias expansion by using single-quotes instad of double quotes