Linux: Fix lowlevelfileio and NativeFileSystem unit tests, enable grunt test-integration task

* Fix `brackets.fs.unlink` when deleting a non-empty directory
* Fix `brackets.fs.makedir` error codes
* Implement `--startup-path` to enable brackets core `grunt test-integration` task
* Remove `user-data-dir` param in `OpenLiveBrowser`
* General clean and replace some posix calls with equivalent glib
* Fix crash when opening dev tools