Add a function to check if the user is root.

[damani42]

Add a helper function to check if the user is root. This function can be
useful for the prompts for example. Modify the prompts made root checked
to use the function instead. Add also the support of Administrator like
a root user.

Fixes: #7031

[faho]

Thank you for your effort!

What I'd really like to see, and what makes the entire thing with the function worthwhile, is to have it used in all of our sample prompts that do this check. E.g. the "debian chroot" prompt has this bit:

fish-shell/share/tools/web_config/sample_prompts/debian_chroot.fish

Lines 22 to 33 in f2d5f95

	switch "$USER"
	case root toor
	echo -n -s "$USER" @ (prompt_hostname) ' ' (set -q fish_color_cwd_root
	and set_color $fish_color_cwd_root
	or set_color $fish_color_cwd) (prompt_pwd) \
	(set_color normal) '# '
	case '*'
	echo -n -s "$USER" @ (prompt_hostname) ' ' (set_color $fish_color_cwd) (prompt_pwd) \
	(set_color normal) '> '
	end

In essence, whenever a prompt checks if the user is "root" anywhere, it should be using that function instead. That includes now using the function whether it used to check for "root" or "toor" or just for "root". The actual coloring and using symbols can stay in the prompt, however.

So: If you're feeling up for it, please check the sample_prompts directory for such uses and replace them with this function.

[damani42]

I think now I have modified all prompts. Do you see something else, I can modify?

[faho]

Thank you very much, merged!