cleanup MAC PDU generation (plus some general cleanup)

The code was very unclear and potentially buggy.
This new version is more robust.

We can waste up to 2 bytes because the last header in the MAC PDU
does not contain a length field and when we request data from RLC
we suppose a 3-bytes MAC header. This might be optimized at some
point, but the benefit would be low.

This commit also contains some general cleanup:
- formatting
- variables' types: let's use 'int' instead of trying to be clever
  by using small types that may generate bugs if the value is
  too big
- remove 'tpc_accumulated' which was globally used for all UEs
  and has no purpose other than logging. We may want to rework
  a bit the TPC machinery at some point. As the code is today
  we may repeatedly send TPC over and over without caring about
  the 3GPP limits, in which case no one knows how the UE is
  supposed to behave: does it clamp the current max value or does
  it accumulate over and over and take the clamped value to compute
  its actual power? If we send a reverse TPC (reduce power instead
  of increase) does it do it immediately or does it have to decrease
  n+1 times if we previously ordered it to increase n times?)

We do not address the problem of prioritizing LCIDs. As of today there
is only one dedicated traffic channel (DTCH), so it's not a problem
at this point.

What has been tested:
- monolithic eNB 5/10/20MHz with one cots UE, TCP/UDP UL/DL. At 20MHz the
  machine used was not capable of keeping up, generating lots of Us
  and Ls when the throughput reaches 60Mb/s. USRP B210 was used.

openair2/LAYER2/MAC/proto.h

@@ -855,7 +855,7 @@ in the DLSCH buffer.
 @param post_padding number of bytes for padding at the end of MAC PDU
 @returns Number of bytes used for header
 */
-unsigned char generate_dlsch_header(unsigned char *mac_header,
+int generate_dlsch_header(unsigned char *mac_header,
                           unsigned char num_sdus,
                           unsigned short *sdu_lengths,
                           unsigned char *sdu_lcids,