Improve handling of subprocess shutdown on MS-Windows.
src/w32proc.c (reader_thread): Do not index fd_info[] with negative
values.
(reader_thread): Exit when cp->status becomes STATUS_READ_ERROR
after WaitForSingleObject returns normally. This expedites reader
thread shutdown when delete_child triggers it.
(reap_subprocess): More accurate commentary for why we call
delete_child only when cp->fd is negative.
src/w32.c (sys_close): Do not call delete_child on a subprocess
whose handle is not yet closed. Instead, set its file descriptor
to a negative value, so that reap_subprocess will call
delete_child on that subprocess when its SIGCHLD arrives. This
avoids closing handles used for communications between sys_select
and reader_thread, which doesn't give sys_select a chance to
notice that the process exited and invoke the SIGCHLD handler for
it.