gio: Add source tags to various GTasks constructed in GLib

This makes them easier to identify when debugging and profiling.

This patch was somewhat less than interesting to write.

https://bugzilla.gnome.org/show_bug.cgi?id=767765

gio/gunixmount.c

@@ -309,6 +309,7 @@ eject_unmount_do (GMount              *mount,
   GSource *timeout;
 
   task = g_task_new (mount, cancellable, callback, user_data);
+  g_task_set_source_tag (task, eject_unmount_do);
   g_task_set_task_data (task, g_strdupv (argv), (GDestroyNotify) g_strfreev);
 
   if (unix_mount->volume_monitor != NULL)