multimedia/ffmpeg port - add "disable network" option

[Александр Ушаков]

Dear, ffmpeg maintainer,

FFmpeg is known for several cross-origin vulnerabilities 
(https://www.cvedetails.com/cve/CVE-2016-1898/ for example) which allow 
to get data from local system even if it processes only local files 
(Theses files may be uploaded to local system from insecure places). 
There is a flag for ffmpeg configure "--disable-network" which 
completely disables ffmpeg interaction with network and gives protection 
from such vulnerabilities (and some others). It will be very useful for 
users who process with ffmpeg only local files to add such an option to 
the ffmpeg port.

-- 
Regards,
Alexander Ushakov