Leave zombie processes once in a while

Environment:
- CentOS release 6.4 (Final) x86_64
- fluentd (0.10.41)
- fluent-plugin-multiprocess (0.1.0)

Log:

```
# /etc/init.d/td-agent restart
Shutting down td-agent: [ OK ]
Starting td-agent: [ OK ]

# ps auxwwf | grep td-agent
root 30277 0.0 0.0 103236 852 pts/0 S+ 23:05 0:00 \_ grep td-agent
td-agent 30190 0.0 0.1 210932 14192 ? Sl 23:05 0:00 /usr/lib64/fluent/ruby/bin/ruby /usr/sbin/td-agent --group td-agent --log /var/log/td-agent/td-agent.log --daemon /var/run/td-agent/td-agent.pid
td-agent 30193 1.1 0.2 236224 21848 ? Sl 23:05 0:00 \_ /usr/lib64/fluent/ruby/bin/ruby /usr/sbin/td-agent --group td-agent --log /var/log/td-agent/td-agent.log --daemon /var/run/td-agent/td-agent.pid
td-agent 30202 1.5 0.0 0 0 ? Z 23:05 0:00 \_ [ruby]
td-agent 30215 1.5 0.2 210932 17016 ? Sl 23:05 0:00 \_ /usr/lib64/fluent/ruby/bin/ruby /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.41/lib/fluent/command/fluentd.rb -i \n type forward\n port 24224 \n -c /etc/td-agent/td-agent_child.conf
td-agent 30218 8.3 0.5 323168 41616 ? Sl 23:05 0:01 \_ /usr/lib64/fluent/ruby/bin/ruby /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.41/lib/fluent/command/fluentd.rb -i \n type forward\n port 24224 \n -c /etc/td-agent/td-agent_child.conf

# /etc/init.d/td-agent restart
Shutting down td-agent: [ OK ]
Starting td-agent: [ OK ]

# ps auxwwf | grep td-agent
root 30404 0.0 0.0 103236 852 pts/0 S+ 23:06 0:00 \_ grep td-agent
td-agent 30320 0.0 0.1 210932 14188 ? Sl 23:06 0:00 /usr/lib64/fluent/ruby/bin/ruby /usr/sbin/td-agent --group td-agent --log /var/log/td-agent/td-agent.log --daemon /var/run/td-agent/td-agent.pid
td-agent 30323 1.3 0.2 236224 21784 ? Sl 23:06 0:00 \_ /usr/lib64/fluent/ruby/bin/ruby /usr/sbin/td-agent --group td-agent --log /var/log/td-agent/td-agent.log --daemon /var/run/td-agent/td-agent.pid
td-agent 30332 1.6 0.2 210932 17016 ? Sl 23:06 0:00 \_ /usr/lib64/fluent/ruby/bin/ruby /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.41/lib/fluent/command/fluentd.rb -i \n type forward\n port 24225 \n -c /etc/td-agent/td-agent_child.conf
td-agent 30338 1.6 0.2 240772 19560 ? Sl 23:06 0:00 | \_ /usr/lib64/fluent/ruby/bin/ruby /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.41/lib/fluent/command/fluentd.rb -i \n type forward\n port 24225 \n -c /etc/td-agent/td-agent_child.conf
td-agent 30345 1.8 0.2 210932 17012 ? Sl 23:06 0:00 \_ /usr/lib64/fluent/ruby/bin/ruby /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.41/lib/fluent/command/fluentd.rb -i \n type forward\n port 24224 \n -c /etc/td-agent/td-agent_child.conf
td-agent 30348 8.8 0.5 323168 39052 ? Sl 23:06 0:01 \_ /usr/lib64/fluent/ruby/bin/ruby /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/fluentd-0.10.41/lib/fluent/command/fluentd.rb -i \n type forward\n port 24224 \n -c /etc/td-agent/td-agent_child.conf
```