[2.4.x] Merge changes to mod_systemd

[notroj]

Merge r1928356, r1937596, r1937597, r1937598, r1937599, r1937600, r1937601, r1937602, r1937635, r1937644, r1937646, r1937647 from trunk:

The .github/workflows/linux.yml change from r1937596 is not included: 2.4.x CI has no reduced-exports job and no pytest target selection for it to hook into, so the new test/modules/arch/linux suite is not run by CI here.

mod_systemd: add missing space

Submitted by: Christian Hesse
Github: closes #557

• test/modules/arch/linux/: New test suite.

• test/pyhttpd/env.py (HttpdTestEnv.set_httpd_env, _clean_path_env):
  Allow a suite to add variables to the environment httpd is started
  with.

• test/pyhttpd/conf/stop.conf.template: Set DefaultRuntimeDir and
  PidFile to match httpd.conf, so that "apachectl -k stop" finds the
  running server.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
GitHub: PR #723

• modules/arch/unix/mod_systemd.c (systemd_monitor): Avoid dividing by a
  zero uptime, and report the idle and busy figures as percentages.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
GitHub: PR #723

• modules/arch/unix/mod_systemd.c (ap_systemd_listen_fds,
  ap_find_systemd_socket): Remember the number of sockets passed, so a
  graceful restart of a socket-activated server works. Drop the
  unchecked getenv("LISTEN_FDS").

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
GitHub: PR #723

• modules/arch/unix/mod_systemd.c (monotonic_usec, systemd_pre_config):
  Send MONOTONIC_USEC with RELOADING=1.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
GitHub: PR #723

• modules/arch/unix/mod_systemd.c (systemd_stopping,
  systemd_post_config): Notify STOPPING=1 on shutdown.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
GitHub: PR #723

• docs/manual/mod/mod_systemd.xml: Socket activation is supported.
  Generated HTML not rebuilt.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
GitHub: closes #723

• test/modules/arch/: Add missing init.py
  files lost in translation from the PR.

Fixes: 4abdd20 (r1937596)

• test/pyhttpd/env.py (HttpdTestEnv.apache_hard_restart, read_pid_file):
  Moved here from the mod_systemd tests, for use by other suites. [skip ci]

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
GitHub: PR #727

mod_systemd: Implement watchdog support:

• modules/arch/unix/mod_systemd.c (watchdog_ping): New.
  (systemd_pre_config, systemd_post_config, systemd_monitor): Send the
  watchdog keep-alive, and warn (AH10621) if WatchdogSec is too short
  to be met.

• docs/manual/mod/mod_systemd.xml: Document watchdog support.

• test/modules/arch/linux/test_006_watchdog.py: New test suite.

• test/modules/arch/linux/env.py (ForegroundServer): New, running httpd
  in the foreground so $WATCHDOG_PID can name it.
  (TransientService): Take extra unit properties.

• test/modules/arch/linux/test_001_notify.py
  (test_systemd_001_09_watchdog): No longer xfail; use a WatchdogSec
  which does not draw AH10621.

• test/modules/arch/linux/README: Describe the watchdog tests.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
GitHub: closes #728

• docs/manual/mod/mod_systemd.xml: Document Type=notify-reload,
  ReloadSignal= and ExecReload=.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
GitHub: PR #729

• test/modules/arch/linux/test_007_notify_reload.py: New test suite for
  Type=notify-reload.

• test/modules/arch/linux/env.py (TransientService): Take the service
  type and whether to configure ExecReload=.
  (TransientService.systemctl): Take a timeout.
  (TransientService.reload, TransientService.rewrite_conf,
  error_log_size, error_log_since, systemd_version): New.

• test/modules/arch/linux/README: Describe the new suite.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
GitHub: PR #729

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