Fix cudu

This fixes the F1 split after the introduction of function pointers in the PDCP layer to more easily handle the monolithic/F1 split.

1. Patch aligns PROTO_AGENT PDCP Ind/RLC Req functions with function pointer definition.
2. Patch unifies and corrects the setting of the function pointer. The following problems are solved through this:
   1. The initialization was handled partly in the enb_config.c, partly in the lte-softmodem.c source files. In particular, the CU's function pointers were set in read_config_and_init() and subsequently overwritten in pdcp_module_init(). This and the whole PDCP initialization is now handled in a central place in lte-softmodem.c. Also, pdcp_module_init() does not set the function pointers anymore but has to be set explicitly to avoid any ambiguity.
   2. The function pointer for DU were not set. This is now handled.
3. Patch: The CU and DU each need only RLC Data Req and PDCP Data Ind, respectively. This patch changes the function pointer setter function so that they can be set independently. CU and DU will not have the function pointers for PDCP Data Ind and RLC Data Req, respectively (this might lead to a segfault, but for instance the CU should never perform a PDCP Data Ind, so this might be seen as beneficial).

@frtabu Would you mind reviewing this? We might run the CI once you are ok with the changes.