New port: sysutils/etc_os-release

Modern Linux systems provide /etc/os-release containing information
about the installed operating system. For example Qt reads said file
to get that information.

For more information on the file, see:
https://www.freedesktop.org/software/systemd/man/os-release.html

Reviewed by: adridg, danfe, kwm
Differential Revision: https://reviews.freebsd.org/D19866