backends: Set error when opening /sys file fails

The caller in clutter really expects an error if fd==-1, so make
sure we set one here. Otherwise we get a nice crash in addition to
the failure to open the /sys file. Also, retry on EINTR.

https://bugzilla.gnome.org/show_bug.cgi?id=784881

src/backends/native/meta-launcher.c

@@ -187,9 +187,22 @@ on_evdev_device_open (const char  *path,
 
   /* Allow readonly access to sysfs */
   if (g_str_has_prefix (path, "/sys/"))
+    {
+      do
         {
           fd = open (path, flags);
-      if (fd >= 0)
+        }
+      while (fd < 0 && errno == EINTR);
+
+      if (fd < 0)
+        {
+          g_set_error (error,
+                       G_FILE_ERROR,
+                       g_file_error_from_errno (errno),
+                       "Could not open /sys file: %s: %m", path);
+          return -1;
+        }
+
       g_hash_table_add (self->sysfs_fds, GINT_TO_POINTER (fd));
       return fd;
     }