udevadm: introduce cat command

This introduces 'udevadm cat' command, that shows udev rules files or
udev.conf, which may be useful for debugging.

Closes #35818.

shell-completion/bash/udevadm

@@ -103,11 +103,13 @@ _udevadm() {
         [TEST_BUILTIN]='-a --action'
         [VERIFY_STANDALONE]='--no-summary --no-style'
         [VERIFY_ARG]='-N --resolve-names --root'
+        [CAT_STANDALONE]='--tldr --config'
+        [CAT_ARG]='--root'
         [WAIT]='-t --timeout --initialized=no --removed --settle'
         [LOCK]='-t --timeout -d --device -b --backing -p --print'
     )
 
-    local verbs=(info trigger settle control monitor test-builtin test verify wait lock)
+    local verbs=(info trigger settle control monitor test-builtin test verify cat wait lock)
     local builtins=(blkid btrfs hwdb input_id keyboard kmod net_driver net_id net_setup_link path_id uaccess usb_id)
 
     for ((i=0; i < COMP_CWORD; i++)); do
@@ -323,6 +325,30 @@ _udevadm() {
             fi
             ;;
 
+        'cat')
+            if __contains_word "$prev" ${OPTS[CAT_ARG]}; then
+                case $prev in
+                    --root)
+                        comps=''
+                        compopt -o dirnames
+                        ;;
+                    *)
+                        comps=''
+                        ;;
+                esac
+            elif [[ $cur = -* ]]; then
+                comps="${OPTS[COMMON]} ${OPTS[CAT_ARG]} ${OPTS[CAT_STANDALONE]}"
+            elif __contains_word "--config" ${COMP_WORDS[*]}; then
+                comps="${OPTS[COMMON]} ${OPTS[CAT_ARG]} ${OPTS[CAT_STANDALONE]}"
+            elif [[ $cur = */* ]]; then
+                comps=$( __get_udev_rules_files )
+                compopt -o dirnames
+            else
+                comps=$( __get_udev_rules_names )
+                compopt -o default
+            fi
+            ;;
+
         'wait')
             if __contains_word "$prev" ${OPTS[WAIT]}; then
                 case $prev in

shell-completion/zsh/_udevadm

@@ -126,6 +126,17 @@ _udevadm_verify(){
         '*::files:_files'
 }
 
+(( $+functions[_udevadm_cat] )) ||
+_udevadm_cat(){
+    _arguments \
+        '(- *)'{-h,--help}'[Show help]' \
+        '(- *)'{-V,--version}'[Show package version]' \
+        '--root=[Operate on catalog hierarchy under specified directory]:directories:_directories' \
+        --tldr'[Skip comments and empty lines.]' \
+        --config'[Show udev.conf.]' \
+        '*::files:_files'
+}
+
 (( $+functions[_udevadm_wait] )) ||
 _udevadm_wait(){
     _arguments \