x11-servers/xorg-server: Fix several security issues

Fix several security issues in x11-servers/xorg-server and slave ports which
ultimately can lead to local privilege escalations if xorg-server is running
privileged.

More info:
https://lists.x.org/archives/xorg-announce/2020-August/003058.html

MFH: 2020Q3 (implicit, security update)
Security: ffa15b3b-e6f6-11ea-8cbf-54e1ad3d6335