🐛 Fix missing execution permission bit in execFile override

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

atom/common/asar/archive.h

@@ -27,6 +27,7 @@ class Archive {
   struct FileInfo {
     FileInfo() : size(0), offset(0) {}
     bool unpacked;
+    bool executable;
     uint32 size;
     uint64 offset;
   };