raise ConfigError when same label appears twice, or no match appears in label

[tagomoris]

This change fixes the 2 <label> related bugs:

no <match> sections in <label> isn't reported

<label @test>
  @type stdout
</label>

This is clearly wrong configuration, and warning "no patterns matched" appears always for @test label.
This change raises configuration error for such configuration.

same label configured twice

<label @test>
  # first
</label>
<label @test>
  # last
</label>

Currently, Fluentd uses the last configuration for same labels without any errors nor warnings.
This configuration is clearly a misconfiguration. So this change is to make to report such configurations as errors.

[tagomoris]

@repeatedly review this please.

[repeatedly]

Good catch!