[SCB-260] Clarify original copyright of some files

[yangbor]

Some files are forked from third parties. Modified those files' headers
to clarify the license and copyright information. Also updated NOTICE
to show their copyright information and origin.

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[coveralls]

Coverage decreased (-0.02%) to 87.292% when pulling 5bd624c on yangbor:master into 25fe20d on apache:master.