Consider an electron application that uses `execFile` to run a script
that lives within the application code base:
```coffee
child_process = require 'child_process'
child_process.execFile __dirname + '/script.sh', (error) ->
throw error if error?
```
An application like this will fail when being packaged in an `asar` with
an following error:
```
Error: spawn EACCES
```
Electron overrides certain `fs` functions to make them work within an
`asar` package. In the case of `execFile`, the file to be executed is
extracted from the `asar` package into a temporary file and ran from
there.
The problem is that during the extraction, the original permissions of
the file are lost.
We workaround this by:
1. Extending `asar.stat` to return whether a file is executable or not,
which is information that's already saved in the `asar` header.
2. Setting execution permissions on the extracted file if the above
property holds true.
Fixes: https://github.com/atom/electron/issues/3512
