childprocess: Fix exec on Windows
childprocess: Fix exec on Windows

Without specifiying verbatim=true, `exec` calls would fail with an error like

 '\"cmd.exe\"' is not recognized as an internal or external command,
 operable program or batch file.

because Libuv would try to escape the double quotes, but no escaping should be done when using `cmd.exe /s /c`

Relevant links:
- http://docs.libuv.org/en/v1.x/process.html#c.uv_process_flags
- https://stackoverflow.com/questions/9866962/what-is-cmd-s-for

zhaozg/luvit,luvit/luvit,zhaozg/luvit,luvit/luvit