Restore code for closing inherited file descriptors in the child.

Restore code for closing inherited file descriptors in the child.

The code closing all file descriptors inherited from the parent in the child
process created by wxExecute() was  removed in r57324 by mistake (probably
due the fact that its meaning was poorly explained) but we still do need to do
this, of course, to avoid descriptor "leaks" (e.g. the parent couldn't really
close any of them).

Restore the code for closing all unneeded file descriptors in the child in
slightly modified form and add a comment pointing to an URL explaining how to
do it better in the future.

Closes #12636.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66153 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775