Remove openair1/SIMULATION/ETH_TRANSPORT/netlink_init.c, and move TUN
interface creation functionality to tun_if.c.

Remove SIMU_ETH target.

Move openair3/RRC/NAS/nas_config.c to common/utils/tun_if.c, and remove
the former from all targets.

- Remove globals, declare where they are used
- Cleanup pdcp_fifo.c
- Remove unnecessary headers from netlink_init.c
- Reformat netlink_init.c

Handle IPv6 (and IPv4v6) PDU sessions by correctly instructing
nas_config() to set up the interface with provided IP addresses.
Note that as of now, IPv6 PDU sessions have not been fully tested.

Remove a useless assert at the gNB; to my understanding the gNB does not
care about the IP session type (at least for IP), as it just
transparently tunnels the IP packets from/to GTP.

I added a comment in the code where to change to request a PDU session
different from IPv4. Currently it is hardcoded to IPv4, and it seems
that a default OAI 5GC config would reject a PDU session type different
from IPv4.

- Read interface flags before setting status to not inadvertently
  overwrite existing flags
- Set interface flag point-to-point, remove multicast (I think a UE is
  not supposed to send anywhere else than to the UPF, so it seems to not
  make sense to declare the interface as multicast)

Implement handling of IPv4 and IPv6 (on the same interface) in
nas_config().

These rules & route are necessary to properly send packets on oaitun_ue1
interface:

1. This forces the packets coming from a subnet different than the UE's
   subnet to go back through oaitun_ue1 rather than via what the default
   route defined on the system (e.g., important if ping from the
   internet arrives).

2. On machines setting net.ipv4.conf.oaitun_ue1.rp_filter=1 (e.g.
   RHEL), this prevents that source filtering for packets coming back is
   applied and those packets be dropped. By default, many hosts have
   rp_filter=2, so no strict source filtering is applied, and it would
   work; on others, this rule prevents source filter dropping.

Prior to this commit, there was a global variable baseNetAddress that
could be set independently through a setter and also through the
configuration module. This baseNetAddress (16 bits IPv4) would then be
complemented with two more bytes in nas_config().

However, this is counter-productive, as not only we have a global
variable that is avoidable (we can give the entire address to
nas_config() directly), but it also would not work with IPv6. Hence,
modify to give nas_config() the entire address. A follow-up commit will
add IPv6 support.

- 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.