zsh only ever offered unit names after `systemctl show`, since
_systemctl_show() was sharing the same completion function as
is-active/status/preset/etc., none of which recognize `-`-prefixed
words at all. This meant `systemctl show --<TAB>` and
`systemctl show --property <TAB>` produced no completions, even
though the same options complete fine before the subcommand word
(bash has always completed both cases correctly).
Give `show` its own completion function that first tries the
existing global option specs (now pulled out into a `_sys_arguments`
array so they can be reused) via _arguments, falling back to unit
name completion for everything else. The unit-name fallback is
factored into a shared _systemctl_non_template_units() helper so it
isn't duplicated between _systemctl_show() and the existing
NONTEMPLATE_UNITS loop.
While doing so, fix pre-existing bugs in that option list which this
change makes newly reachable (and, for the filters below, newly
relevant) after `show`:
- `-P` was documented as a bare `--privileged[Acquire privileges
before execution]` flag, which systemctl does not have. `-P NAME`
is actually short-only and equivalent to `--value --property=NAME`
(src/systemctl/systemctl.c); it now completes property names the
same way `-p`/`--property` do, and is grouped next to them instead
of sitting in `--privileged`'s old, unrelated slot.
- `-p`/`--property`, `-t`/`--type`, and `--state` all lacked the `*`
repeatability prefix, so completion stopped offering each after its
first use even though systemctl accumulates all three
(parse_property_argument()/parse_types_argument()/
parse_states_argument(), all via strv_consume()). `_loginctl`
already uses the correct `'*'{-p+,--property=}...` form for -p.
-t/--type and --state are both documented as filters that apply to
`show` specifically (man/systemctl.xml), same as -p/--property.
- `--value`, documented specifically for `show` ("When showing
properties, only print the value"), was missing from the zsh
completion entirely, though bash already completes it.
- `-M`/`--machine=` (OPTION_COMMON_MACHINE) was likewise missing,
despite directly affecting `show`'s bus transport the same way the
already-present `-H`/`--host` does; `_loginctl`/`_busctl` already
have the matching `_sd_machines`-backed spec.
Also switch both `_arguments` call sites to splice `_sys_arguments`
as `"$_sys_arguments[@]"` rather than bare, matching the prevailing
quoted-array idiom used by five other completions in this directory
(_systemd-hwdb, _userdbctl, _systemd-id128, _updatectl,
_systemd-sysupdate) and avoiding the unquoted form's behavior of
silently eliding any element that becomes empty.
Fixes#37463
