nspawn: Add try-{host,guest} journal link modes

--link-journal={host,guest} fail if the host does not have persistent
journalling enabled and /var/log/journal/ does not exist. Even worse, as there
is no stdout/err any more, there is no error message to point that out.

Introduce two new modes "try-host" and "try-guest" which don't fail in this
case, and instead just silently skip the guest journal setup.

Change -j to mean "try-guest" instead of "guest", and fix the wrong --help
output for it (it said "host" before).

Change systemd-nspawn@.service.in to use "try-guest" so that this unit works
with both persistent and non-persistent journals on the host without failing.

https://bugs.debian.org/770275