This commit splits filling Requested-Service-Unit, Used-Service-Unit and
QoS-Information into their own helper functions for better readibility,
and then partially reverts 125740727e,
where lots of AVPs were left out of INITIAL_REQUEST messagesi during the
changes made.
After looking through 3GPP TS 32.299 and rfc4006, it seems expected to
send Requested-Service-Unit only during INITIAL_REQUEST, and
Used-Service-Unit during UPDATE_REQUEST, so that part is kept.
However, I am not able to find clear indications that AVPs such as QoS
Information and RAT-Type should not be sent during INITIAL_REQUEST.
So, since we have the info, better set it already during
INITIAL_REQUEST, since the OCS may want to grant different resources
based on that information if available too.