This branch series implements the usage of the F1 Setup Request
internally at the gNB, even if CU and DU run in the same process. Until
now, the F1 Setup Request contained struct fields to pass IP
configuration to the F1AP module; this information is not necessary in
monolithic, nor should it be relevant when passing the information in
F1 Setup Request.

Therefore, this commit moves all IP connection-related information out
of the F1 Setup Request and into a separate structure instead.  In order
to give the F1AP module the necessary connection information, the
DU_REGISTER_REQ message is introduced that combines the F1 Setup Request
and IP configuration, such that the F1AP module can setup the SCTP
connection and then send the F1 Setup Request message.

In a later commit, there will be a callback for F1 Setup Request at MAC
to simply register the DU at the CU. The monolithic callback will simply
forward the F1 Setup Request The F1AP callback will look up the IP
configuration and create DU_REGISTER_REQ message containing IP
configuration and F1 Setup Request, which is then handled by the F1AP
module. To this end, a separate function in gnb_config.c allows to read
IP information from configuration.