MFH: r482030 r482658 r482928 r483316

Enable XCB option by default

Rationale:

• XCB offers a comfortable screen capturing function via ffmpeg -f x11grab
• In the default package libxcb is installed as a dependency of libX11 anyway, hence no need to disable it for ffmpeg specifically

PR: 231215
Reported by: vermaden@interia.pl

multimedia/ffmpeg: unbreak ALSA=off after r468844

https://ffmpeg.org/pipermail/ffmpeg-devel/2018-September/234516.html

PR: 232498

multimedia/ffmpeg: Do not set DATADIR twice

PR: 232637

Unbreak build on 12-BETA after OpenSSL 1.1.1 import into base

PR: 232707
Submitted by: fluffy

Approved by: ports-secteam (riggs)