Pass arguments that look like env variables as env to systemd-nspawn

Pass arguments that look like env variables as env to systemd-nspawn

The kernel passes unknown parameters as environment variables to pid1.
Let's do the same for systemd-nspawn. Of course we don't know what is
known and unknown so let's take advantage of the fact that kernel cmdline
arguments are (usually) lower case and environment variables are (usually)
upper case and use that to determine whether to pass something as an argument
or an environment variable.