1. wxProcess changes to make capturing subprocess output easier (and more

1. wxProcess changes to make capturing subprocess output easier (and more
   reliable), now works in both sync and async modes
2. wxSafeYieldBug() corrected, wxWindowDisabler which is now used in it
   added and documented
3. exec sample updated to illustrate capturing the subprocess output
4. wxStreamBase::IsOk() added
5. wxInputStream::Eof() added and non-blocking Eof() implementation in
   wxPipeInputStream used by wxExecute

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@6400 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775