adefault: fix default $default to `false` (`0` was supposed to be printable but enforces int-comparison with == which is usually not desirable)

type handling: automatically treat *_id as 'u', remove redundant *_id-rules from */inlinks.php
support expiration and explicit invalidation ("logout") of sessions
remove spurious `degree_id`
support `valid` in sessionslist
joins2expression():
 - support strings as input
 - support table aliases in $using
 - generate default JOIN rule USING primary key
sql_delete: support joins ("USING" clause in sql) to allow filtering on colums in joined tables

prune_sessions: invalidate expired sessions; handle transactions and persistentvars (wip)