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
# The first commit's message is:
Simplify default fish_prompt
No need for the set_color caching now that it's a builtin.
Also simplify the 3 classic prompts in fish_config's sample_prompts set.
Currently fish doesn't recognize toor as special. However, it's likely
that on BSD systems, fish shell will be used on toor, not on root (toor
is an intentionally existing account to use more advanced shell on, like
shell).
