Don't use "cp -i" in the example WAL archive_command.

This is a dangerous example to provide because on machines with GNU cp,
it will silently do the wrong thing and risk archive corruption.  Worse,
during the 9.0 cycle somebody "improved" the discussion by removing the
warning that used to be there about that, and instead leaving the
impression that the command would work as desired on most Unixen.
It doesn't.  Try to rectify the damage by providing an example that is safe
most everywhere, and then noting that you can try cp -i if you want but
you'd better test that.

In back-patching this to all supported branches, I also added an example
command for Windows, which wasn't provided before 9.0.