Create /etc/os-release file.

Each boot, regenerate /var/run/os-release based on the currently running
system. Create a /etc/os-release symlink pointing to this file (so that this
doesn't create a new reason /etc can not be mounted read-only).

This is compatible with what other systems do and is what the sysutil/os-release
port attempted to do, but in an incomplete way. Linux, Solaris and DragonFly all
implement this natively as well. The complete standard can be found at
https://www.freedesktop.org/software/systemd/man/os-release.html

Moving this to the base solves both the non-standard location problem with the
port, as well as the lack of update of this file on system update.

Bump __FreeBSD_version to 1300060

PR: 238953
Differential Revision: https://reviews.freebsd.org/D22271