Only restore executable permissions on extracted files

We need to check that the create_system is unix, otherwise the
external_attr does not contain unix permissions. It also doesn't make
much sense to restore any flags other than executable.

package_control/package_manager.py

@@ -11,6 +11,7 @@
 
 from concurrent import futures
 from io import BytesIO
+from stat import S_IXUSR
 from threading import RLock
 from urllib.parse import urlencode
 
@@ -54,6 +55,8 @@
     'https://sublime.wbond.net/repositories.json'
 ])
 
+ZIP_UNIX_SYSTEM = 3
+
 
 class PackageManager:
 
@@ -1053,8 +1056,10 @@ def _extract_zip(self, name, zf, src_dir, dest_dir, extracted_files=None):
                 with zf.open(info) as fsrc, open(dest, 'wb') as fdst:
                     shutil.copyfileobj(fsrc, fdst)
 
-                # restore file mode
-                os.chmod(dest, info.external_attr >> 16)
+                # Restore executable permissions
+                if (info.create_system == ZIP_UNIX_SYSTEM
+                        and (info.external_attr >> 16) & S_IXUSR):
+                    os.chmod(dest, os.stat(dest).st_mode | S_IXUSR)
 
             except OSError as e:
                 if e.errno == 5 or e.errno == 13:  # permission denied