Don't restrict ourselves to a "max" fd when closing fds before exec()

Don't restrict ourselves to a "max" fd when closing fds before exec()
when we have a way to get an actual list of all open fds from the OS.

Fixes issue #21618: The subprocess module would ignore fds that were
inherited by the calling process and already higher than POSIX resource
limits would otherwise allow.  On systems with a functioning /proc/self/fd
or /dev/fd interface the max is now ignored and all fds are closed.