Add a function to check if the user is root.

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

doc_src/cmds/fish_is_root_user.rst

@@ -0,0 +1,31 @@
+.. _cmd-fg:
+
+fish_is_root_user - check if the current user is root
+=====================================================
+
+Synopsis
+--------
+
+::
+
+    fish_is_root_user
+
+Description
+-----------
+
+``fish_is_root_user`` will check if the current user is root. It can be useful
+for the prompt to display something different if the user is root, for example.
+
+
+Example
+-------
+
+A simple example:
+
+::
+
+    function example --description 'Just an example'
+        if fish_is_root_user
+            do_something_different
+        end
+    end