Windows args quoting

`bp::detail::windows::build_args("echo", {"trailing backspace and spaces\\"})`
returns:
`echo "trailing backspace and spaces\"`
Note that the closing quote that is inserted by boost::process is escaped.
According to [this microsoft blog post](https://blogs.msdn.microsoft.com/twistylittlepassagesallalike/2011/04/23/everyone-quotes-command-line-arguments-the-wrong-way/) it should be:
`echo "traling backspace and spaces\\"`

Also:
`bp::detail::windows::build_args("echo", {"\\\" backslash in front of quote"})`
returns:
`echo "\\" backslash in front of quote"`
According to the blog it should be:
`echo "\\\" backslash in front of quote"`

As a workaround, i am assembling and escaping the command string myself as described in the above blog post instead of using the `bp::args()` interface.