MT#4261 Proper number handling on subscribers

Outsource the handling of the primary number from
the subscriber controller to a util sub called
update_subscriber_numbers.
It also takes care to reuse a number that has
subscriber_id set to NULL which caused the original
problem in the ticket.
MT#4261 number handling on subscriber update

Actually use update_subscriber_numbers when updating a subscriber
and not only when creating one. Also handle all neccessary side effects.
Now we can switch our primary number to a number which has been
inactive (that is: did not belong to a subscriber) without an update error.