systemctl: restore --failed (#5198)

'systemctl --failed' is an extremely common operation and it's nice to have
a shortcut for it.

Revert "man: don't document systemctl --failed" and add the option back to
systemctl's help and shell completion scripts.

This reverts commit 036359ba8d.

shell-completion/bash/systemctl.in

@@ -126,7 +126,7 @@ _systemctl () {
                [STANDALONE]='--all -a --reverse --after --before --defaults --force -f --full -l --global
                              --help -h --no-ask-password --no-block --no-legend --no-pager --no-reload --no-wall --now
                              --quiet -q --privileged -P --system --user --version --runtime --recursive -r --firmware-setup
-                             --show-types -i --ignore-inhibitors --plain'
+                             --show-types -i --ignore-inhibitors --plain --failed'
                       [ARG]='--host -H --kill-who --property -p --signal -s --type -t --state --job-mode --root
                              --preset-mode -n --lines -o --output -M --machine'
         )