Fix bug #18420 with deadlocks communicating with subprocess on MS-Windows.

Fix bug #18420 with deadlocks communicating with subprocess on MS-Windows.

 src/w32.c (fcntl): Support O_NONBLOCK fcntl on the write side of
 pipes.
 (sys_write): When a write to a non-blocking pipe returns ENOSPC,
 set errno to EAGAIN instead, to allow the caller to retry the
 write after some waiting.  Fixes deadlocks when Emacs exchanges a
 lot of data through the pipe.