This reverts commit b17b688c

We used to set 'does_nr' of an UE if we detect ENDC supported in the
UE capabilities.

Then we activate NR measurements if 'does_nr' is true.

The problem is that if the eNB is not connected to a gNB but the UE
reports some NR measurements (because a gNB is running somewhere near)
then the eNB will crash when starting the switch to NR.

A (quick) solution is to set 'does_nr' only if there is a gNB connected to
the eNB.

Maybe not the best solution. To be changed if needed.

- a new section in the configuration file to select security algorithms,
  with new code to deal with it
- cleanup CG-ConfigInfo: specs seem to indicate that we must not
  add mcg_RB_Config; the gNB has to deal with that
- as a consequence, modify fill_default_rbconfig() that is called in
  every cases and needs security and bearer parameters

The new section in the configuration file looks like:

    security = {
      # preferred ciphering algorithms
      # the first one of the list that an UE supports in chosen
      # valid values: nea0, nea1, nea2, nea3
      ciphering_algorithms = ( "nea0", "nea2" );

      # preferred integrity algorithms
      # the first one of the list that an UE supports in chosen
      # valid values: nia0, nia1, nia2, nia3
      integrity_algorithms = ( "nia0" );
    };

The code is forced to use nea2, no matter what the UE supports.

After 2^18 PDCP packets, it will fail to work (we don't use HFN yet).

These limitations will be fixed in later commits.

The existing security function was not reused, because it does too
much memory allocation and initializes the security context at each
ciphering. So here comes nr_pdcp_security_nea2_cipher(). And also
the ciphering is done inplace. To be changed if necessary.

- compute correct kgNB
- propagate properly NR security capabilities of an UE
  (involves: rrc, s1ap, x2ap)

Wireshark (version 3.3.2) was not happy.
https://asn1.io/asn1playground/ was not happy.

Let's make them happy.

So then everyone is happy.

This commit contains work done by Eurecom and by Benetel.

dedicated_DRB was not set properly when the NSA UE sends
ReconfigurationComplete so the wrong code was executed below
(we had dedicated_DRB == 0). Let's put a value that does
nothing (-1 seems fine).

This commit introduces several enhancements in X2AP and RRC to deal
with ENDC. The main goal is to deal with UE's on/off and with various
errors.

The following work is included:

    - DC prep and DC overall X2 timers
    - X2 messages for UE release and DC timers' timeout

Open problems:

    - what to do when the UE sends an scg Failure? This commit removes
      the UE. And this removal may not be correctly done
    - UE removal in the gNB's PHY layer (a next commit is coming,
      but it is hackish and will need a rework)