Add $EUID and use it in fish_is_root_user

Fixes #8866

doc_src/language.rst

@@ -1416,6 +1416,10 @@ Fish also provides additional information through the values of certain environm
 
    the current username. This variable can be changed.
 
+.. ENVVAR:: EUID
+
+   the current effective user id, set by fish at startup. This variable can be changed.
+
 .. envvar:: version
 
    the version of the currently running fish (also available as ``FISH_VERSION`` for backward compatibility).

share/functions/fish_is_root_user.fish

@@ -2,6 +2,10 @@
 # else if the user is root.
 
 function fish_is_root_user --description "Check if the user is root"
+    if test "$EUID" = 0 2>/dev/null
+        return 0
+    end
+
     if contains -- $USER root toor Administrator
         return 0
     end

src/env.cpp

@@ -294,6 +294,7 @@ void env_init(const struct config_paths_t *paths, bool do_uvars, bool default_pa
     // so we work around it by resetting $USER.
     // TODO: Figure out if that su actually checks if username == "root"(as the man page says) or
     // UID == 0.
+    vars.set_one(L"EUID", ENV_GLOBAL, to_string(static_cast<unsigned long long>(geteuid())));
     uid_t uid = getuid();
     setup_user(uid == 0);