Added clearing of cpu_affinity after process spawn.

This fixes unwanted/incorrect cpu_affinity use on dead worker processes
respawn.  While this is not ideal, it's expected to be better when previous
situation where multiple processes were spawn with identical CPU affinity
set.

Reported by Charles Chen.

src/os/unix/ngx_process_cycle.c

@@ -371,6 +371,8 @@ ngx_start_worker_processes(ngx_cycle_t *cycle, ngx_int_t n, ngx_int_t type)
 
         ngx_pass_open_channel(cycle, &ch);
     }
+
+    cpu_affinity = 0;
 }