fixup a problem in unit filter handling of strange children

Right now, many places in the code construct unit filters with
children that it ignores, for instance [message] results in
unit_filters which may have [option] children. There's no easy
way to filter those out right now, so messages of spurious
children are relegated to "debug config" channel rather than
a more prominent place (since there are likely to be many
false reports of a problem in the current arrangement.)
Merge branch 'master' of git://github.com/wesnoth/wesnoth