Fix wxKill(wxSIGNONE) always returning true for child processes in wxMSW.

Fix wxKill(wxSIGNONE) always returning true for child processes in wxMSW.

The fact that a handle to a process can be opened doesn't mean that the
process is still running. In fact, for a child process that we store a handle
for ourselves we will always be able to open (another copy of the) handle even
if it already terminated.

Check for the process termination using WaitForSingleObject() instead in both
normal and wxSIGNONE cases.

Also simplify the code by not using GetExitCodeProcess() at all as we don't
need the process exit code.

Closes #2834.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65493 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775