Log unused section configuration as warn level

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@tagomoris Could you check this?

[tagomoris]

default value false is against to comment.

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other wise refers to "if this element is not used". I will add more line.

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Apply review. Please check again.

@sonots Could you check this too?

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Here is logged example:

2015-07-13 17:54:45 +0900 [info]: using configuration file: <ROOT>
  <source>
    type secure_forward
    self_hostname server.fqdn.example.com
    shared_key secret_string
    secure no
    private_key_passphrase xxxxxx
    <user>
      username test
      password test_pass
      <foo>
        a b
      </foo>
    </user>
    <test>
      foo bar
    </test>
  </source>
  <match debug.**>
    type stdout
    <test>
      c d
    </test>
  </match>
  <fil>
    type foo
  </fil>
  <matc foo.**>
    type bar
  </matc>
</ROOT>
2015-07-13 17:54:45 +0900 [warn]: section <foo> is not used in <user> of secure_forward plugin
2015-07-13 17:54:45 +0900 [warn]: section <test> is not used in <source> of secure_forward plugin
2015-07-13 17:54:45 +0900 [warn]: section <test> is not used in <match debug.**>
2015-07-13 17:54:45 +0900 [warn]: section <fil> is not used in <ROOT>
2015-07-13 17:54:45 +0900 [warn]: section <matc> is not used in <ROOT>

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If no problem, I will merge it today.

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If you ok, please merge this.