Fix hang when deleting a pipe process

Fix hang when deleting a pipe process

* src/w32.h (FILE_DONT_CLOSE): New flag.
* src/w32.c (sys_close): Don't close descriptors used to read from
the pipe process.  Leave the FILE_DONT_CLOSE flag set in the
descriptor's info.
(register_aux_fd): Set the FILE_DONT_CLOSE flag in the
descriptor's info.
* src/w32proc.c (reader_thread): When exiting normally, close the
file descriptor used to read from a pipe process.  (Bug#52414)