tree-wide: change --kill-who to --kill-whom

getopt allows non-ambiguous abbreviations, so backwards-compat is maintained, and
people can use --kill-who (or even shorter abbreviations). English is flexible,
so in common speach people would use both forms, even if "whom" is technically
more correct. The advantage of using the longer form in the code is that we
effectively allow both forms, so we stop punishing people who DTGCT¹, but still
allow people to use the spoken form if they prefer.

1. Do the gramatically correct thing

shell-completion/bash/loginctl

@@ -42,7 +42,7 @@ _loginctl () {
     local -A OPTS=(
         [STANDALONE]='--all -a --help -h --no-pager --version
                       --no-legend --no-ask-password -l --full --value'
-        [ARG]='--host -H --kill-who --property -p --signal -s -M --machine
+        [ARG]='--host -H --kill-whom --property -p --signal -s -M --machine
                       -n --lines -o --output -P'
     )
 
@@ -52,7 +52,7 @@ _loginctl () {
                 _signals
                 return
                 ;;
-            --kill-who)
+            --kill-whom|--kill-who)
                 comps='all leader'
                 ;;
             --host|-H)

shell-completion/bash/machinectl

@@ -36,7 +36,7 @@ _machinectl() {
     local -A OPTS=(
         [STANDALONE]='--all -a -l --full --help -h --no-ask-password --no-legend --no-pager --version --value
                       --mkdir --read-only --force -q --quiet'
-        [ARG]='--host -H --kill-who -M --machine --property -p --signal -s --uid -E --setenv -n --lines
+        [ARG]='--host -H --kill-whom -M --machine --property -p --signal -s --uid -E --setenv -n --lines
                       -o --output --verify --format --max-addresses'
     )
 
@@ -64,7 +64,7 @@ _machinectl() {
                 _signals
                 return
                 ;;
-            --kill-who)
+            --kill-whom|--kill-who)
                 comps='all leader'
                 ;;
             --host|-H)

shell-completion/bash/systemctl.in

@@ -128,7 +128,7 @@ _systemctl () {
                              --help -h --no-ask-password --no-block --legend=no --no-pager --no-reload --no-wall --now
                              --quiet -q --system --user --version --runtime --recursive -r --firmware-setup
                              --show-types --plain --failed --value --fail --dry-run --wait'
-        [ARG]='--host -H --kill-who --property -p --signal -s --type -t --state --job-mode --root
+        [ARG]='--host -H --kill-whom --property -p --signal -s --type -t --state --job-mode --root
                              --preset-mode -n --lines -o --output -M --machine --message --timestamp --check-inhibitors'
     )
 
@@ -156,7 +156,7 @@ _systemctl () {
                 comps='fail replace replace-irreversibly isolate
                                        ignore-dependencies ignore-requirements flush'
                 ;;
-            --kill-who)
+            --kill-whom|--kill-who)
                 comps='all control main'
                 ;;
             --root)