Use alter table to rename tables

For some strange reason using "rename table" causes an exception in
system/database/DB_driver.php:query() because
it trys to call num_rows() on a boolean. Using "alter table" works.

According to the php manpage mysqli::query() it should return a boolean
too, but whatever...

Signed-off-by: Florian Pritz <bluewind@xinu.at>
migration 12: remove dependencies on users table again

When using external authentication databases (ldap, fluxbb, ..) the
users table is empty so adding the constraints won't work.

Signed-off-by: Florian Pritz <bluewind@xinu.at>