Client rc file migration_gtk2/gtk3_from_2_5 use wrong defaults
gui_gtk3_migrated_from_2_5 is used as the default when loading
migration_gtk2_from_2_5, and vice versa.
Probably this doesn't bite very much, since the hardcoded defaults are the same. There's probably a sequence of starting 2.6+ Gtk2 and Gtk3 for the first time where a full-screen preference gets clobbered, but I haven't worked out what it is.