The GTP module is common for 4G and 5G. It hardcodes the functions to
call when creating S1U and NGU tunnels. The S1U case is both applicable
for LTE and NR (NSA), which until now was not a problem as the PDCP data
forwarding functions where called pdcp_data_req. The issue is that in this
branch, the NR PDCP functions have changed to nr_pdcp_data_req_drb().

When setting up a GTP tunnel, we now pass the PDCP data forwarding
functions into GTP, since the caller knows whether it is in 4G or 5G,
but GTP itself does not.

Merge remote-tracking branch 'origin/develop-handle-reconfigurationWithSync' into integration_2023_w13

In the UE, we wish to handle ^C in the UE to send a message before
terminating, so we allow custom wait handlers to implement such
functionality. Use itti_wait_tasks_unblock() to unblock
itti_wait_tasks_end().

Also, implement itti_wait_tasks_end() using semaphores, as signals are
not async-signal-safe (and therefore should not be used in a signal
handler).