Added option --check-inhibitors for non-tty usage

As described in #2680, systemctl did ignore inhibitors if it is not
attached to a tty to allow scripts to ignore inhibitors automatically.
This pull request preserves this behavior but allows scripts to
explicit check inhibitors if required.

The new parameter '--check-inhibitors=yes' enables this feature.
The old parameter '-i'/'--ignore-inhibitors' was deprecated in favor
of '--check-inhibitors=no', the default behaviour can be specified
with '--check-inhibitors=auto'.
The new parameter is also described in the documentations and shell
completions found here.

shell-completion/bash/systemctl.in

@@ -125,9 +125,9 @@ _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 --system --user --version --runtime --recursive -r --firmware-setup
-                             --show-types -i --ignore-inhibitors --plain --failed --value --fail --dry-run --wait'
+                             --show-types --plain --failed --value --fail --dry-run --wait'
         [ARG]='--host -H --kill-who --property -p --signal -s --type -t --state --job-mode --root
-                             --preset-mode -n --lines -o --output -M --machine --message --timestamp'
+                             --preset-mode -n --lines -o --output -M --machine --message --timestamp --check-inhibitors'
     )
 
     if __contains_word "--user" ${COMP_WORDS[*]}; then
@@ -179,6 +179,9 @@ _systemctl () {
             --timestamp)
                 comps='pretty us µs utc us+utc µs+utc'
                 ;;
+            --check-inhibitors)
+                comps='auto yes no'
+                ;;
         esac
         COMPREPLY=( $(compgen -W '$comps' -- "$cur") )
         return 0