storage: add 'storagectl' command-line tool

CLI for inspecting and using storage providers. Scans
/run/systemd/io.systemd.StorageProvider/ (or the user-mode equivalent)
for AF_UNIX sockets and talks to each one over Varlink. Verbs:
"volumes" lists volumes across all providers, "templates" lists
supported creation templates, "providers" lists the endpoints
themselves.

Also installed as a mount.storage helper, so
'mount -t storage PROVIDER:VOLUME /mnt' (or 'mount -t storage.<fstype>'
to put a fresh filesystem on a block volume) acquires the volume and
mounts it. Ships with bash/zsh completions and a man page.

shell-completion/bash/meson.build

@@ -36,6 +36,7 @@ foreach item : [
         ['portablectl',         'ENABLE_PORTABLED'],
         ['resolvectl',          'ENABLE_RESOLVE'],
         ['run0',                ''],
+        ['storagectl',          ''],
         ['systemd-analyze',     ''],
         ['systemd-cat',         ''],
         ['systemd-cgls',        ''],

shell-completion/bash/storagectl

@@ -0,0 +1,74 @@
+# shellcheck shell=bash
+# storagectl(1) completion                            -*- shell-script -*-
+# SPDX-License-Identifier: LGPL-2.1-or-later
+#
+# This file is part of systemd.
+#
+# systemd is free software; you can redistribute it and/or modify it
+# under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation; either version 2.1 of the License, or
+# (at your option) any later version.
+#
+# systemd is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+# General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with systemd; If not, see <https://www.gnu.org/licenses/>.
+
+__contains_word () {
+    local w word=$1; shift
+    for w in "$@"; do
+        [[ $w = "$word" ]] && return
+    done
+}
+
+_storagectl() {
+    local i verb comps
+    local cur=${COMP_WORDS[COMP_CWORD]} prev=${COMP_WORDS[COMP_CWORD-1]}
+
+    local -A OPTS=(
+        [STANDALONE]='-h --help --version --no-pager --no-legend --no-ask-password
+                      --system --user'
+        [ARG]='--json'
+    )
+
+    if __contains_word "$prev" ${OPTS[ARG]}; then
+        case $prev in
+            --json)
+                comps=$( storagectl --json=help 2>/dev/null )
+                ;;
+        esac
+        COMPREPLY=( $(compgen -W '$comps' -- "$cur") )
+        return 0
+    fi
+
+    if [[ "$cur" = -* ]]; then
+        COMPREPLY=( $(compgen -W '${OPTS[*]}' -- "$cur") )
+        return 0
+    fi
+
+    local -A VERBS=(
+        [STANDALONE]='volumes templates providers help'
+    )
+
+    for ((i=0; i < COMP_CWORD; i++)); do
+        if __contains_word "${COMP_WORDS[i]}" ${VERBS[*]} &&
+                ! __contains_word "${COMP_WORDS[i-1]}" ${OPTS[ARG]}; then
+            verb=${COMP_WORDS[i]}
+            break
+        fi
+    done
+
+    if [[ -z ${verb-} ]]; then
+        comps=${VERBS[*]}
+    elif __contains_word "$verb" ${VERBS[STANDALONE]}; then
+        comps=''
+    fi
+
+    COMPREPLY=( $(compgen -W '$comps' -- "$cur") )
+    return 0
+}
+
+complete -F _storagectl storagectl