Improve boot-time gathering

Simplify Emacs proper by using Gnulib’s boot-time module
instead of doing it all by hand.  This should port Emacs
better to obscurish hosts, as Bruno Haible has merged the
best of Emacs’s and Gnulib’s boot-time gathering.
* lib/boot-time-aux.h, lib/boot-time.c, lib/boot-time.h:
* lib/readutmp.h, m4/readutmp.m4: New files, copied from Gnulib.
* admin/merge-gnulib (GNULIB_MODULES): Add boot-time.
* configure.ac: Do not check for utmp.h;
the boot-time module now does this.
(BOOT_TIME_FILE): Remove; no longer used.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* src/filelock.c [__FreeBSD__]: Do not include <sys/sysctl.h>.
[HAVE_UTMP_H]: Do not include utmp.h.
Include boot-time instead: boot-time does the work now.
(BOOT_TIME) [HAVE_ANDROID && !ANDROID_STUBIFY]: Don’t undef.
(WTMP_FILE): Don’t define.
(boot_time, boot_time_initialized, get_boot_time_1, get_boot_time):
Remove.
(get_boot_sec): New function that simply calls Gnulib get_boot_time.
(lock_file_1, current_lock_owner): Use get_boot_sec instead
of get_boot_time.

m4/readutmp.m4

@@ -0,0 +1,117 @@
+# readutmp.m4 serial 28
+dnl Copyright (C) 2002-2023 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_READUTMP],
+[
+  AC_REQUIRE([gl_SYSTEMD_CHOICE])
+
+  dnl Set READUTMP_LIB to '-lsystemd' or '', depending on whether use of
+  dnl systemd APIs is possible and desired (only the systemd login API, here).
+  dnl AC_LIB_LINKFLAGS_BODY would be overkill here, since few people install
+  dnl libsystemd in non-system directories.
+  READUTMP_LIB=
+  if test "$SYSTEMD_CHOICE" = yes; then
+    AC_CHECK_HEADER([systemd/sd-login.h])
+    if test $ac_cv_header_systemd_sd_login_h = yes; then
+      AC_CACHE_CHECK([for libsystemd version >= 254],
+        [gl_cv_lib_readutmp_systemd],
+        [gl_save_LIBS="$LIBS"
+         LIBS="$LIBS -lsystemd"
+         AC_LINK_IFELSE(
+           [AC_LANG_PROGRAM([[
+              #include <stdint.h>
+              #include <systemd/sd-login.h>
+              ]], [[
+              uint64_t st;
+              sd_session_get_start_time ("1", &st);
+              ]])
+           ],
+           [gl_cv_lib_readutmp_systemd=yes],
+           [gl_cv_lib_readutmp_systemd=no])
+         LIBS="$gl_save_LIBS"
+        ])
+      if test $gl_cv_lib_readutmp_systemd = yes; then
+        AC_DEFINE([READUTMP_USE_SYSTEMD], [1],
+          [Define if the readutmp module should use the systemd login API.])
+        READUTMP_LIB='-lsystemd'
+      fi
+    fi
+  fi
+  AC_SUBST([READUTMP_LIB])
+
+  gl_PREREQ_READUTMP_H
+])
+
+# Prerequisites of readutmp.h and boot-time-aux.h.
+AC_DEFUN_ONCE([gl_PREREQ_READUTMP_H],
+[
+  dnl Persuade utmpx.h to declare utmpxname
+  AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
+
+  AC_CHECK_HEADERS_ONCE([utmp.h utmpx.h])
+  if test $ac_cv_header_utmp_h = yes || test $ac_cv_header_utmpx_h = yes; then
+    dnl Prerequisites of lib/readutmp.h and lib/readutmp.c.
+    AC_CHECK_FUNCS_ONCE([utmpname utmpxname])
+    AC_CHECK_DECLS([getutent],,,[[
+/* <sys/types.h> is a prerequisite of <utmp.h> on FreeBSD 8.0, OpenBSD 4.6.  */
+#include <sys/types.h>
+#ifdef HAVE_UTMP_H
+# include <utmp.h>
+#endif
+]])
+    utmp_includes="\
+AC_INCLUDES_DEFAULT
+#ifdef HAVE_UTMPX_H
+# include <utmpx.h>
+#endif
+#ifdef HAVE_UTMP_H
+# if defined _THREAD_SAFE && defined UTMP_DATA_INIT
+   /* When including both utmp.h and utmpx.h on AIX 4.3, with _THREAD_SAFE
+      defined, work around the duplicate struct utmp_data declaration.  */
+#  define utmp_data gl_aix_4_3_workaround_utmp_data
+# endif
+# include <utmp.h>
+#endif
+"
+    AC_CHECK_MEMBERS([struct utmpx.ut_user],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmp.ut_user],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmpx.ut_name],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmp.ut_name],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmpx.ut_type],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmp.ut_type],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmpx.ut_pid],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmp.ut_pid],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmp.ut_tv],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmpx.ut_host],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmp.ut_host],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmpx.ut_id],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmp.ut_id],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmpx.ut_session],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmp.ut_session],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmpx.ut_exit],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmp.ut_exit],,,[$utmp_includes])
+
+    AC_CHECK_MEMBERS([struct utmpx.ut_exit.ut_exit],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmpx.ut_exit.e_exit],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmp.ut_exit.e_exit],,,[$utmp_includes])
+
+    AC_CHECK_MEMBERS([struct utmpx.ut_exit.ut_termination],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmpx.ut_exit.e_termination],,,[$utmp_includes])
+    AC_CHECK_MEMBERS([struct utmp.ut_exit.e_termination],,,[$utmp_includes])
+  fi
+
+  AC_CHECK_HEADERS_ONCE([sys/param.h])
+  dnl <sys/sysctl.h> requires <sys/param.h> on OpenBSD 4.0.
+  AC_CHECK_HEADERS([sys/sysctl.h],,,
+    [AC_INCLUDES_DEFAULT
+     #if HAVE_SYS_PARAM_H
+     # include <sys/param.h>
+     #endif
+    ])
+  AC_CHECK_FUNCS([sysctl])
+
+  AC_CHECK_HEADERS_ONCE([OS.h])
+])