On POSIX systems you wouldn't use 'b'. But on Windows
(http://msdn.microsoft.com/en-us/library/96ayss4b%28VS.80%29.aspx )
the 'b' is required for a binary mode.
In my opinion Lua programs shouldn't have to care if it is on Windows
or POSIX, so I'd call this a bug in the io library for not
interpreting the mode flags. Or alternatively I would express this as,
you can't use binary data with io.popen in a portable way because you
can't reliably pass 'b', which Windows needs.
