Fix putenv race conditions with undefined behavior.

Do all putenv calls before Emacs creates any threads.
Use a safer way to modify the TZ environment variable in the
presence of multiple threads.  For further thread-safety,
prefer localtime_r and gmtime_r to localtime and gmtime,
and prefer struct tm's tm_gmtoff (if available) to calling
both localtime_r and gmtime_r.
* configure.ac (LOCALTIME_CACHE): Remove.
We needn't worry about SunOS 4 any more; Sun dropped support in 2003.
All uses of LOCALTIME_CACHE removed.  This simplifies the fix.
(tzalloc): Add check for this function.
* admin/merge-gnulib (GNULIB_MODULES): Add time_r, since Emacs now
calls localtime_r and gmtime_r directly.
* src/dbusbind.c (Fdbus__init_bus): Move xputenv call from here ...
(init_dbusbind): ... to this new function.
* src/emacs.c (main) [HAVE_DBUS]: Call it before creating threads.
* src/xterm.c (x_term_init): Move xputenv call from here ...
(init_xterm): ... to this new function.
* src/emacs.c (main) [USE_GTK]: Call it before creating threads.
* src/editfns.c (HAVE_TM_GMTOFF): Default to false.
(dump_tz_string): New constant.
(init_editfns): Use it.  This centralizes the dump_tz stuff.
Call set_time_zone_rule here, so that its xputenv is done
before Emacs goes multithreaded.
(mktime_z) [!HAVE_TZALLOC]: New function, which is typically
thread-safe enough for Emacs.
(format_time_string, Fdecode_time, Fcurrent_time_string)
(Fcurrent_time_zone):
Prefer localtime_r and gmtime_r, which are more thread-safe, to
localtime and gmtime.  Remove now-unnecessary calls to block_input.
(tm_gmtoff): New static function.
(Fdecode_time, Fcurrent_time_zone): Use it.
(Fencode_time): Use mktime_z, for better thread-safety.
(set_time_zone_rule): Now static.  Rewrite to be mostly thread-safe,
i.e., not quite thread-safe but good enough for Emacs typical usage.
Do not reclaim storage that is in the environment; let it leak.
Always call tzset, since localtime_r does not.
* src/emacs.c (dump_tz, Fdump_emacs) [HAVE_TZSET]: Remove dump_tz stuff.
This is now done in init_editfns.
* src/systime.h (mktime_z, timezone_t, tzalloc, tzfree) [!HAVE_TZALLOC]:
New macros and declarations, for platforms lacking tzalloc & friends.