Proposed fixes to sitemap.php

1. Explicitly set the action parameter for the form. (fixes Bug #1252278)
2. WT_Filter::postBool returns null if the posted variable doesn't exist (maybe use FILTER_NULL_ON_FAILURE flag?) 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 is updated the cached value isn't