Allow proper shutdown by adding libevent's loop-break to SIGINT handler

This may be related to known issues with PID file, but SIGINT terminates entire application without allowing shutdown sequence to complete. 

```C
  /* enter the event loop */
  if (event_base_loop(main_base, 0) != 0) {  
    retval = EXIT_FAILURE;
  } 
  // Everything below is unreachable
```

Ideally, `sig_handler` would interrupt event's main loop, and allow memcached to free resources properly.