Document xorg-server CVEs 2017-12176 through 2017-12187.

While here replace the SO-AND-SO part in the description of the previous xorg-server entry[1], with the Alan Coopersmith who send the announce mail to xorg-announce@ mailing list.

[1] entry: 4f8ffb9c-f388-4fbd-b90f-b3131559d888