Unreachable code in examples.

[Stephen Hemminger <stephen@networkplumber.org>]

Building DPDK with clang and -Wunreachable-code shows a common pattern
across many examples. They all have a for(;;) loop but no exit condition
for the loop. 

There are two options here.
 - do real signal and cleanup (preferred)
 - remove the dead code.

I am busy but if someone wants to spend 1/2 day doing and testing
this great.

[3666/3960] Compiling C object examples/dpdk-pipeline.p/pipeline_main.c.o
../examples/pipeline/main.c:188:2: warning: code will never be executed [-Wunreachable-code]
  188 |         rte_eal_cleanup();
      |         ^~~~~~~~~~~~~~~
1 warning generated.
[3687/3960] Compiling C object examples/dpdk-mp_client.p/multi_process_client_server_mp_mp_client_client.c.o
../examples/multi_process/client_server_mp/mp_client/client.c:272:2: warning: code will never be executed [-Wunreachable-code]
  272 |         rte_eal_cleanup();
      |         ^~~~~~~~~~~~~~~
1 warning generated.
[3715/3960] Compiling C object examples/dpdk-timer.p/timer_main.c.o
../examples/timer/main.c:126:2: warning: code will never be executed [-Wunreachable-code]
  126 |         (void) lcore_mainloop(NULL);
      |         ^      ~~~~~~~~~~~~~~~~~~~~
1 warning generated.
[3735/3960] Compiling C object examples/dpdk-service_cores.p/service_cores_main.c.o
../examples/service_cores/main.c:226:2: warning: code will never be executed [-Wunreachable-code]
  226 |         rte_eal_cleanup();
      |         ^~~~~~~~~~~~~~~
1 warning generated.
[3763/3960] Compiling C object examples/dpdk-efd_node.p/server_node_efd_efd_node_node.c.o
../examples/server_node_efd/efd_node/node.c:394:2: warning: code will never be executed [-Wunreachable-code]
  394 |         rte_eal_cleanup();
      |         ^~~~~~~~~~~~~~~
1 warning generated.
[3788/3960] Compiling C object examples/dpdk-rxtx_callbacks.p/rxtx_callbacks_main.c.o
../examples/rxtx_callbacks/main.c:348:2: warning: code will never be executed [-Wunreachable-code]
  348 |         rte_eal_cleanup();
      |         ^~~~~~~~~~~~~~~
1 warning generated.
[3816/3960] Compiling C object examples/dpdk-skeleton.p/skeleton_basicfwd.c.o
../examples/skeleton/basicfwd.c:218:2: warning: code will never be executed [-Wunreachable-code]
  218 |         rte_eal_cleanup();
      |         ^~~~~~~~~~~~~~~