This is somewhat similar to emulators/linux_base-c7, except:
- It's from Ubuntu Focal
- It's one port instead of many small ones
- It installs under /usr/local/compat/linux, not /compat/linux
Obviously it needs some adjusting for various mounts, changing
the compat.linux.emul_path etc; I'll do that later on.
Note that this is not a proper chroot or jail. Everything
normally configured by the package triggers is missing, because
we don't execute them, only unpack. I think this shouldn't cause
any problems, but if it does, it's to be addresssed at install
time.