Improve boot-time gathering

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.