Proposed fixes to sitemap.php

1. Explicitly set the action parameter for the form.
2. WT_Filter::postBool returns null if the posted variable doesn't exist
(I can't see why this happens) which is the case if the checkbox is
unchecked. This then means that WT::Tree->preference does a get not a
set.
3. When setting a gedcom preference, although the database was updated
the cached value wasn't