systemctl,verbs: Introduce SYSTEMD_OFFLINE environment variable

systemctl,verbs: Introduce SYSTEMD_OFFLINE environment variable

A lot of code references the `running_in_chroot()` function; while
I didn't dig I'm pretty certain this arose to deal with situations
like RPM package builds in `mock` - there we don't want the `%post`s
to `systemctl start` for example.

And actually this exact same use case arises for
[rpm-ostree](https://github.com/projectatomic/rpm-ostree/)
where we implement offline upgrades by default; the `%post`s are
always run in a new chroot using [bwrap](https://github.com/projectatomic/bubblewrap).

And here's the problem: bwrap creates proper mount roots, so it
passes `running_in_chroot()`, and then if a script tries to do
`systemctl start` we get:
`System has not been booted with systemd as init system (PID 1)`
but that's an *error*, unlike the `running_in_chroot()` case where we ignore.

Further complicating things is there are real world RPM packages
like `glusterfs` which end up invoking `systemctl start`.

A while ago, the `SYSTEMD_IGNORE_CHROOT` environment variable was
added for the inverse case of running in a chroot, but still wanting
to use systemd as PID 1 (presumably some broken initramfs setups?).

Let's introduce a `SYSTEMD_OFFLINE` environment variable for cases like
mock/rpm-ostree so we can force on the "ignore everything except preset" logic.
This way we'll still not start services even if mock switches to use nspawn or
bwrap or something else that isn't a chroot.

We also cleanly supercede the `SYSTEMD_IGNORE_CHROOT=1` which is now spelled
`SYSTEMD_OFFLINE=0`.  (Suggested by @poettering)

Also I made things slightly nicer here and we now print the ignored operation.