Fixes issue #8052: The posix subprocess module's close_fds behavior was
suboptimal by closing all possible file descriptors rather than just
the open ones in the child process before exec().

It now closes only the open fds when it is possible to safely determine what
those are.
diff --git a/configure b/configure
index d6bd2bf..10ee46c 100755
--- a/configure
+++ b/configure
@@ -6147,7 +6147,7 @@
 sys/audioio.h sys/xattr.h sys/bsdtty.h sys/event.h sys/file.h sys/loadavg.h \
 sys/lock.h sys/mkdev.h sys/modem.h \
 sys/param.h sys/select.h sys/sendfile.h sys/socket.h sys/statvfs.h \
-sys/stat.h sys/termio.h sys/time.h \
+sys/stat.h sys/syscall.h sys/termio.h sys/time.h \
 sys/times.h sys/types.h sys/uio.h sys/un.h sys/utsname.h sys/wait.h pty.h \
 libutil.h sys/resource.h netpacket/packet.h sysexits.h bluetooth.h \
 bluetooth/bluetooth.h linux/tipc.h spawn.h util.h