feat: list systemd services in one round trip and control them, once for every driver

Listing a host's services and starting or stopping one is the same on every
host that runs systemd, so the command, its parse, the check of a unit name
and the reading of an action's exit status live here once, and a driver
supplies only the transport (napalm-linux#7, napalm-proxmox#6):

- SYSTEMD_SERVICES_COMMAND: one read-only POSIX sh line. list-unit-files, then
  one systemctl show over every loaded service unit (Id, Names, LoadState,
  ActiveState, SubState, UnitFileState, MainPID), and is-enabled only for
  generated units, whose boot state lives in a SysV script's rc links. Framed;
  [no-systemd] when /run/systemd/system is missing. Replaces an is-enabled and
  a show per unit: 0.8 s instead of 6 s on a 180-unit Ubuntu host.
- parse_systemd_services(): loaded units except not-found, plus installed unit
  files that are not loaded; no templates, no aliases (also not the ones older
  systemd lists as "enabled"). enabled = UnitFileState enabled or
  enabled-runtime, read from systemctl show and never from list-unit-files'
  second column, which has had a preset column after it since systemd 245.
  A report whose end is missing raises ValueError, so a list cut short never
  reads as services that went away; a host without systemd raises
  SystemdUnavailable, a NotImplementedError, so a driver can fall back.
- unit_name() / service_action_command() / parse_action_result(): template
  instances, dots, colons and \xHH escapes accepted; a bare template, a leading
  "-" and anything a shell reads refused. The action runs as
  "timeout 45 systemctl --no-ask-password <action> -- <unit>.service" with its
  exit status printed after it; only that status decides, 124 is not called
  done, and terminal colour codes are dropped. The marker also keeps the output
  from ever being empty, which a transport that retries on an empty answer
  would take as a reason to run the action twice.
- SystemdServicesMixin, in the template form: get_services() and
  manage_service() are concrete, _run_service_command(command, *, privileged,
  timeout) is the driver's hook. Mixed in by the drivers whose host runs
  systemd, not by OSDriver.

Version 2.2.0.

README.md

@@ -79,6 +79,15 @@ everywhere, so the command and its parse are concrete here and a driver supplies
 `_run_kernel_facts_command`. `OSDriver` does not carry it — a Windows host is an OS driver
 too, and `hasattr(driver, "get_kernel_facts")` has to stay truthful.
 
+`SystemdServicesMixin` (`get_services`, `manage_service`) is mixed in the same way, by
+the drivers whose host runs systemd. Listing the services, checking a unit name and
+reading an action's exit status are the same on every such host, so they are concrete
+here, and a driver supplies only `_run_service_command(command, *, privileged, timeout)`
+— how a command reaches its host and how it gains root there. The listing is one round
+trip (`list-unit-files` plus one `systemctl show` over every loaded unit) instead of an
+`is-enabled` and a `show` per unit. A host without systemd raises `SystemdUnavailable`,
+a `NotImplementedError`, so a driver can fall back to another init system.
+
 A function class may use the **template form** — public method concrete, the
 device-specific part a `_hook` declared under `if TYPE_CHECKING` — *when the base
 genuinely does work* on the result: normalising, sorting, validating, or orchestrating