Correctly handle SIGINT/stop signal in nr-softmodem

• Correctly handle SIGINT, so that the nr-softmodem stops regularly
• Implement abortTpool() to stop the complete Tpool (the previous abortTpool() function has been renamed to abortTpoolJob())
• Implement abortNotifiedFIFO() to unblock an existing FIFO (the previous abortNotifiedFIFO() function has been renamed to abortNotifiedFIFOJob())
• Improve the Tpool documentation markup, see thread-pool.md
• Correctly stop the USRP: it is possible to start the nr/lte-softmodem immediately after it has been stopped, without the need to wait: should solve #519 (closed)
• Some memory fixes: on L1, for NFAPI PUCCH PDUs, and a big memory leak for UP traffic in UL at the GTP

executables/nr-softmodem.c

@@ -833,16 +833,14 @@ int main( int argc, char **argv ) {
 
   /* release memory used by the RU/gNB threads (incomplete), after all
    * threads have been stopped (they partially use the same memory) */
-  for (int inst = 0; inst < NB_gNB_INST; inst++) {
-    //free_transport(RC.gNB[inst]);
-    phy_free_nr_gNB(RC.gNB[inst]);
-  }
-
   for (int inst = 0; inst < NB_RU; inst++) {
-    kill_NR_RU_proc(inst);
     nr_phy_free_RU(RC.ru[inst]);
   }
 
+  for (int inst = 0; inst < NB_gNB_INST; inst++) {
+    phy_free_nr_gNB(RC.gNB[inst]);
+  }
+
   pthread_cond_destroy(&sync_cond);
   pthread_mutex_destroy(&sync_mutex);
   pthread_cond_destroy(&nfapi_sync_cond);