- refactor functions to make them shorter
- provide doxygen strings
- open socket for operation once instead of in each function
- use named constant (from enum) to differentiate interface UP/DOWN
- Linux kernel defines name length as IFNAM_SIZE, so use that for
  interface name length. Also, it seems no null-byte is needed.

I initially planned to entirely remove the netmask configuration.
However, in specific configurations, e.g. in 4G Rfsim with feMBMS,
routing does not seem to work anymore using these commands:

  iperf -B 10.0.2.2 -s -u -i1
  iperf -c 10.0.2.2 -B 10.0.2.1 -u -t 30 -b 2.00M -i1

Therefore, we simplify by hardcoding the netmask to /24, which allows
the above to work.

Remove the broadcast address from the UE, as logically, a UE has nothing
to broadcast: it can only directly talk to the core. Additionally, at
least as of now, the UE interface is shown as "POINTOPOINT", so
broadcasting is not possible.

Remove unused header includes, as well as the test program (which does
not do much, and certainly does not test in the way we use the
interfaces).

Remove nas_config_mbms() since nas_config_mbms() and nas_config() do the
same

The routing table is temporarily removed in this commit and introduced
in one of the next commits.

Does the same change in the _mbms() functions as in the previous commit.

Also, simplify logic in netlink_init_mbms_tun(): directly use the id in
the interface. This requires a slight ID change in the users, which is
also done in this commit.

Instead of implicitly using a hardcoded prefix "oaitun_" interface
name, move that out to the callers of these functions to make it clear.
The ID still remains outside.

The "sister functions" nas_config_mbms()/netlink_init_tun_mbms() will do
the same change in the next commit.

Comparing both functions, it seems that nas_config_mbms_s1() sets the
same parameters, assuming that in nas_config_mbms(), we set
- (UE_NAS_USE_TUN || ENB_NAS_USE_TUN) yielding true -> should yield the
  same interface name
- netmask is hardcoded to 255.255.255.0
- baseNetAddress is hardcoded 10.0

So replace the call with nas_config_mbms() by setting parameters
accordingly.

- Remove unused NAS_config(): It does most of what the highly similar
  nas_config() does.
- Remove unused set_gateway()
- Remove unused blocking_NAS_config()
- Remove unused functions getNetMask()
- Remove unused broadcast functionality: functions setBroadcastAddress()
  and getBroadcastAddress(), as well as global variable broadcastAddr
  are not used in consumers. Remove it.
- Make functions static

I don't know about any user of the kernel modules. It is effectively
legacy code we carry around, for no good reason. Remove it for the
moment. If we ever need it, we can revert.

Remove netlink code, and the PDCP_USE_NETLINK macro, as there is no
known use, it makes the code more complicated, and is simply "technical
debt".

In 4G PDCP, a subtle change has been introduced, in pdcp_module_init()
of pdcp.c. Looking at the diff of this commit, it can be seen that there
is now a new "else-if (ENB_NAS_USE_TUN_W_MBMS_BIT)" branch that is under
what was the "if (PDCP_USE_NETLINK)" branch, while the code fragment was
actually inside the "else" branch of PDCP_USE_NETLINK. This seems to be
necessary: previously, if we did not use noS1 mode and we don't use any
TUN (i.e., the eNB), the else block would set up this interface. It is
important that this interface is brought up, because if we omit it,
RFsim will not work. It seems to me that this interface will regularly
create some traffic; without it, RFsim will be blocked, and the UE
cannot connect.

In short, it seems to be important to have the MBMS interface, which
injects traffic in the RFsim side, which will unblock RFsim. The change
in pdcp_module_init() preserves the total changes of the if-else
(PDCP_USE_NETLINK).

Remove function pointers that were used in the past to implement the
F1(-U) interface in 4G, i.e., to have different functions to handle
split/monolithic operations. As we don't have F1 in 4G anymore (see
commit 9dc22822), we don't need the
function pointers anymore.

On that occasion, rename the 5G functions by prepending nr_ to them;
this allows to refactor the interface in 5G further (which is not the
focus of this changeset).

Integration: `2024.w30`

See merge request oai/openairinterface5g!2877

* !2830 NR feature set update
* !2868 Fix support for USRP UHD Versions => 4.5.0.0
* !2870 fix(docker): base image for target oai-gnb-aw2s shall be jammy
* !2874 Fix memory leaks in gNB PDU Session Setup Request processing
* !2866 RRC: reject PDU sessions if security is not active
* !2829 Automatic update of USRP RX gain during initial search using --agc option
* !2878 USRP: add a T trace to record the TX IQ data on antenna 0
* !2633 Handle F1 Reset at DU, and ensure state re-init on F1Setup
* !2864 Fix an assert in power control
* !2754 Introduce gold sequence cache

Harmonize nr_gold_pusch() and nr_gold_pdsch(), as the gold sequence
calculation for PUSCH and PDSCH is the same.