copy: use btrfs reflinking only whe we know we copy full files
nspawn: properly unset arg_link_journal_try, when --link-journal= is specified
nspawn: don't link journals in ephemeral mode
nspawn: allow spawning ephemeral nspawn containers based on the root file system of the OS

This works now:

        # systemd-nspawn -xb -D / -M foobar

Which boots up an ephemeral container, based on the host's root file
system. Or in other words: you can now run the very same host OS you
booted your system with also in a container, on top of it, without
having it interfere. Great for testing whether the init system you are
hacking on still boots without reboot the system!
nspawn: when booting in ephemeral mode, append random token to machine name

Also, when booting up an ephemeral container of / use the system
hostname as default machine name.

This way specifiyng -M is unnecessary when booting up an ephemeral
container, while allowing any number of ephemeral containers to run from
the same tree.
man: fedora 21 has been release, suggest 21 as fedora version in example yum command line
update TODO