Increase FD_SETSIZE on Windows to support more subprocesses

Earlier versions of Emacs were limited to at most 32 subprocesses or
network connections on Windows due to the 64-object limit imposed by
WaitForMultipleObjects.  To overcome this, a simple waiting thread
pool is implemented, allowing Emacs to efficiently wait on up to
2048 objects.

Each thread in the pool can wait on up to 63 objects, and a total of
32 threads are used, together with the main thread, to expand the
waiting capability.  This enables Emacs to support approximately 1024
subprocesses, which is comparable to the 'pty' method on GNU/Linux
when using the default FD_SETSIZE of 1024.

To minimize overhead, the threads remain active instead of being
frequently created and destroyed, reducing unnecessary system
resource consumption.  Idle threads can be terminated after a
period of inactivity to free up memory.

* src/w32.h (FD_SETSIZE): Change from 64 to 2048.
* src/w32.c (term_ntproc): Call 'free_wait_pool' to free waiting
threads.
* src/w32proc.c (WFO_ABANDONED, WFO_TIMEOUT, WFO_FAILED)
(WFO_MAX_WAIT): New macros.
(wait_objects_context, wait_objects_pool, wait_objects_info):
New structures for managing the thread pool.
(wait_objects_pool, wait_objects_info): New static variables for
managing the thread pool.
(wait_objects_thread, start_wait_objects, stop_wait_objects)
(end_wait_and_return, shrink_wait_pool, free_wait_pool):
New functions for waiting and managing the thread pool.
(wait_for_objects, msg_wait_for_objects): New functions as
replacements for WaitForMultipleObjects and
MsgWaitForMultipleObjects.
(wait_pid): Use 'wait_for_objects' and new macros.
(sys_select): Make some variables static to avoid stack
allocation.  Use 'wait_for_objects', 'msg_wait_for_objects', and
the new macros.

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