Remove configure's --without-sync-input option.

Remove configure's --without-sync-input option.

When auditing signal-handling in preparation for cleaning it up,
I found that SYNC_INPUT has race conditions and would be a real
pain to fix.  Since it's an undocumented and deprecated
configure-time option, now seems like a good time to remove it.
Also see <http://bugs.gnu.org/11080#16>.
* configure.ac (SYNC_INPUT, BROKEN_SA_RESTART): Remove.
* admin/CPP-DEFINES (BROKEN_SA_RESTART, SA_RESTART): Remove.
* etc/TODO (Make SYNC_INPUT the default): Remove, as the code now
behaves as if SYNC_INPUT is always true.
* src/alloc.c (_bytes_used, __malloc_extra_blocks, _malloc_internal)
(_free_internal) [!DOUG_LEA_MALLOC]: Remove decls.
(alloc_mutex) [!SYSTEM_MALLOC && !SYNC_INPUT && HAVE_PTHREAD]:
(malloc_hysteresis):
(check_depth) [XMALLOC_OVERRUN_CHECK]:
(MALLOC_BLOCK_INPUT, MALLOC_UNBLOCK_INPUT):
(__malloc_hook, __realloc_hook, __free_hook, BYTES_USED)
(dont_register_blocks, bytes_used_when_reconsidered)
(bytes_used_when_full, emacs_blocked_free, emacs_blocked_malloc)
(emacs_blocked_realloc, reset_malloc_hooks, uninterrupt_malloc):
[!SYSTEM_MALLOC && !SYNC_INPUT]:
Remove. All uses removed.
(MALLOC_BLOCK_INPUT, MALLOC_UNBLOCK_INPUT): Use a different
implementation, one that depends on whether the new macro
XMALLOC_BLOCK_INPUT_CHECK is defined, not on whether SYNC_INPUT
is defined.
* src/atimer.c (run_timers, handle_alarm_signal):
* src/keyboard.c (pending_signal, poll_for_input_1, poll_for_input)
(handle_async_input, process_pending_signals)
(handle_input_available_signal, init_keyboard):
* src/nsterm.m (ns_read_socket):
* src/process.c (wait_reading_process_output):
* src/regex.c (immediate_quit, IMMEDIATE_QUIT_CHECK):
* src/sysdep.c (emacs_sigaction_init) [SA_RESTART]:
(emacs_write):
* src/xterm.c (XTread_socket):
Assume SYNC_INPUT.
* src/conf_post.h (SA_RESTART) [IRIX6_5]: Do not #undef.
* src/eval.c (handling_signal): Remove.  All uses removed.
* src/lisp.h (ELSE_PENDING_SIGNALS): Remove.
All uses replaced with the SYNC_INPUT version.
(reset_malloc_hooks, uninterrupt_malloc, handling_signal):
Remove decls.
* src/sysdep.c, src/syssignal.h (main_thread) [FORWARD_SIGNAL_TO_MAIN_THREAD]:
Now static.

Fixes: debbugs:12450