edit MariaDB license info so that GitHub recognizes it

GitHub uses a library called Licensee to identify a project's license

type. It shows this information in the status bar and via the API if it

can unambiguously identify the license.

This commit modifies a few of MariaDB's docs so that Licensee is able

to recognize the repository's license type. It renames the README's

"License" section to "Licensing" and renames COPYING.thirdparty to

THIRDPARTY.

These changes allow Licensee to bypass both files when it

scans the repo for license files, which thus allows Licensee to

successfully identify the license type of MariaDB as GPL 2.0.