When the program exits it has to stop the streaming of the USRP.
The function exit_fun is supposed to do that. When quitting with
control+c (very common case) this function is not called. The
code is very unclear there, so let's add a stop_rf in the RU,
as there is already a start_rf.

If we don't call trx_end_func, then at the next run the USRP
device may be in an unstable state and behave improperly.

If the program crashes then the USRP device may be in an
unstable state. The only solution to this problem is to reset
the USRP device.

Maybe there is a way to clean the state of the device when we
open it, before we start using it. Sort of a cleanup before
use. That could be a better solution to "bad state after program
crash".

What has been tested:
- monolithic eNB only

- remove spaces at the end of lines
- remove useless dead code
  use: git show -p <this commit> -w
  to see it clearly

With value 4 when connecting one UE and doing some downlink
iperf TCP there is a crash. Probably due to the multiple wrong
RA detected leading to uplink failure of fake UE that never
sends Msg3 (because, well, there is no other UE).

This is another problem that will be fixed at some point.

Anyway, this value 4 fails. Let's put back 3.

Doing airplane mode off to connect a cots UE followed by airplane mode on
to disconnect it gives very bad results (bad disconnection with lots of
UL failures followed by lots of logs in the eNB). It used to work properly
after the work done by Xu Bo.

It turns out that in 2018.w04 things were still working okay. But
in 2018.w05 no.

So I ran:

    git diff 2018.w04 2018.w05

And I checked all the modifications that I thought were relevant
(everything related to the UE, oaisim, if4 and fapi has not been checked).

This commits takes back the version of 2018.w04.

The modifications in openair2/RRC/LITE/rrc_eNB.c are necessary, I think.

The other modifications may not be necessary or even wrong. To be checked
at some point.

The following compilation did not work anymore:

    ./build_oai --eNB -w USRP -r Rel10 -c

(Rel8 still does not work.)

- add function free_td() -> complements init_td() [seems to not be used, added
  for completeness]
- add function free_td8() -> undoes init_td8() [free memory of 8-bit LLR Turbo
  decoder]
- add function free_td16() -> undoes init_td16() [free memory of 16-bit LLR
  Turbo decoder]
- change function free_tdavx216() -> undoes init_tdavx216() [free memory of
  16-bit LLR Turbo decoder, AVX2 version]
- add free_lte_top() -> frees memory allocated by init_lte_top()
- change free_ul_ref_sigs() to set freed pointers to NULL
- add method free_transport() -> frees memory of ULSCH/DLSCH transport channels
- use the above functions when stopping/restarting the lte-softmodem

this methods free the memory that is allocated by the functions
phy_init_RU() and phy_init_lte_eNB()

As reported by Emad Alizade:

    According to "Issue255 256 257 paging reesta release" that has been
    merged in develop version, we have a question: In rrc_eNB_free_UE()
    function only all ulsch related memory of user has been cleaned, but
    I think not only ulsch memory but also dlsch memory must be cleaned.
    I tested the latest develop version and with repetition UE attach-detach
    procedures we find that the dlsch memory has not been cleaned and after
    repeat this sequence (45 times) assertion with cause UE_id!=-1 (no free
    or exiting dlsch_context, dci_tools.c: fill_dci_and_dlsch() ) occurred
    and no UE will be attached to system.

The fixes in this commit are from Emad Alizade.