Update SOA record after dynamic update (fixes #181)

This PR fixes issue #181: the SOA record is now automatically updated after a record update through dynamic_update.php. This required moving the set_timezone function from toolbox.inc.php (which requires an active session) to record.inc.php (which doesn't require an active session, and is currently the only place in which set_timezone is being used anyway).

Tested on my own setup and works.