#9351: set_defaults on subparser is no longer ignored if set on parent.

Before, if a default was set on the parent parser, any default for that
variable set via set_defaults on a subparser would be ignored.  Now
the subparser set_defaults is honored.

Patch by Jyrki Pullianinen.
Merge: #9351: set_defaults on subparser is no longer ignored if set on parent.
#18853: Fix resource warning in shlex's __main__ section.

Report and original fix by Vajrasky Kok.
Merge: #18853: Fix resource warning in shlex's __main__ section.
Issue #17401: document closefd in io.FileIO docs and add to repr

closefd was documented in the open docs but not the matching FileIO
class documented. Further, closefd, part of the core state for the
object was not shown.

In review it was noted that the open docs are a little confusing about
the interaction between closefd and paths, so tweaked them at the same
time.