Follow-on to 9dbe2e15ad9db3b2be53e46bc5f67d382fb4089d for #1999.
Fix an issue where siblings() by itself throws an error.  The problem is that
Theme_View::siblings() passes a null offset and limit to the callback which
gets passed down to search::search_within_album, which creates its query
in raw SQL and doesn't check for a null offset/limit.  We want a reasonable
limit on the size of the set here (and 1000 is probably wayyy too high so
lower that to 100) so amend get_siblings to stop using default parameters
and actually check the inputs.

Author: Bharat Mediratta <bharat@menalto.com>
Date:   Sat Feb 9 14:53:34 2013 -0500

    Extend siblings callbacks to take a $limit and an $offset for navigating
    large sibling sets.  Useful for the thumbnav module since we don't want to
    iterate a thousand siblings to find the one we care about.  Fixes #1999.