Close monitoring thread handles as soon as possible in wxMSW.

Don't leave the handles of all threads used for monitoring the child processes
open until the main process termination, rather close them as soon as the
monitored process terminates.

This ensures that we don't accumulate potentially unbounded number of open
handles if we keep launching new child processes. An even better idea would be
to actually use a single thread for monitoring all of them, but this fix is
much simpler and should fix the most acute problem for now.

Closes #16123.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76199 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775