Merge pull request #2577 from xmcclure/bugzilla-37834

Recognize OS X file "locked" state (fixes #37834)

Quick fix for https://bugzilla.xamarin.com/show_bug.cgi?id=37834

IOLayer is_file_writable/System.IO.File.GetAttributes on posix systems looks only at the unix file permissions and does not consider special OS flags. This leads to an easily end-user-triggered bug on OS X if a mono app (the bug was observed in Xamarin Studio) uses GetAttribute on a file which has been "Locked" using the checkbox in Finder Get Info. This patch recognizes the "uchg" flag for non-writability which Finder uses.