workaround to fix EACCES in linux

The problem with EACCES in linux is this:

When you run `myapp.exe` generated by `nw-pkg`, the contents of `myapp.nw` appear in `/tmp/.org.chromium.Chromium.xxxxxx/`

`$ ls /tmp/.org.chromium.Chromium.eEYwZV
client.js  index.html  main  package.json  script.js`

However, `main` is not executable, and that's why it has EACCES.

I don't know what is creating this tmp folder and if it's actually unzipping these files from myapp.nw, otherwise I would modify that to include a `chmod +x main`.  `unzip myapp.nw` unpacks the files with the correct permissions.

In this patch, the correct permissions are set on the exe before spawning it as a child process.