Merge r1393976, r1393997, r1484554, r1528032, r1528034, r1614821, r1618579, r1618588...

Merge r1393976, r1393997, r1484554, r1528032, r1528034, r1614821, r1618579, r1618588, r1802251, r1840554, r1840555, r1872761, r1872763 from trunk:

* modules/arch/unix/mod_systemd.c: New module.

Submitted by: Jan Kaluza <jkaluza redhat.com>

* modules/arch/unix/mod_systemd.c (systemd_pre_mpm): Simplify code
  slightly; no functional change.
  Use AP_DECLARE_MODULE.

* configure.in: Simplify/fix systemd detection: move later to fix
  autoconf warnings; define HAVE_SYSTEMD rather than using CPPFLAGS.

* server/listen.c: Use HAVE_SYSTEMD for systemd #define.

* modules/arch/unix/config5.m4: Update systemd headers check.

Don't link support/* against systemd libs.

* Makefile.in: Link httpd using $(HTTPD_LIBS).

* configure.in: Add systemd to HTTPD_LIBS rather than LIBS.  Add PCRE
  libs to HTTPD_LIBS.

* configure.in, acinclude.m4: Move systemd check to APACHE_CHECK_SYSTEMD
  and use pkg-config where available.

mod_systemd: Add IdleShutdown - number of seconds in idle-state after which
httpd is shutdown. This is useful in a combination with socket activation.
Add mod_systemd documentation.

mod_systemd: Use AP_SIG_GRACEFUL instead of SIGWINCH.

Fix overriding ExtendedStatus to "off" with mod_systemd loaded, and
give more feedback to systemd during a reload.

* modules/arch/unix/mod_systemd.c (systemd_pre_config): New
  function; tell systemd the service is reloading here.
  (systemd_pre_mpm): Don't set ap_extended_status here, do nothing
  if ExtendedStatus is off.
  (register_hooks): Register pre_config hook.

* modules/arch/unix/mod_systemd.c (systemd_post_config): Fix systemd
  service getting stuck reloading if "ExtendedStatus off" is
  configured; regression in r1802251.

PR: 62697

* modules/arch/unix/mod_systemd.c (systemd_pre_mpm, systemd_monitor):
  Ignore sd_notify{,f} failure cases as currently recommended by the
  systemd API docs.

* modules/arch/unix/mod_systemd.c: Remove IdleShutdown feature which
  was buggy per sf's feedback in 2.4.x backport proposal, and would
  probably be more appropriate outside this module anyway.

* modules/arch/unix/config5.m4: Don't override enable_systemd, fixing
  --enable-systemd=static per covener's suggestion in
  <CALK=YjPrN644NtLROwYPyBeCpev-GHSAD2J-Z2hupLymZ0op-g@mail.gmail.com>
  Also fix the APACHE_MODULE() usage; disable the module by default
  for the "all" modules selection.

PR: 57632

Submitted by: jkaluza, jorton, nd
Reviewed by: jorton, jim, elukey

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1873519 13f79535-47bb-0310-9956-ffa450edef68