In some containers unshare() is made unavailable entirely. Let's deal
with this that more gracefully and disable our sandboxing of services
then, so that we work in a container, under the assumption the container
manager is then responsible for sandboxing if we can't do it ourselves.
Previously, we'd insist on sandboxing as soon as any form of BindPath=
is used. With this change we only insist on it if we have a setting like
that where source and destination differ, i.e. there's a mapping
established that actually rearranges things, and thus would result in
systematically different behaviour if skipped (as opposed to mappings
that just make stuff read-only/writable that otherwise arent').
(Let's also update a test that intended to test for this behaviour with
a more specific configuration that still triggers the behaviour with
this change in place)
Fixes: #13955
(For testing purposes unshare() can easily be blocked with
systemd-nspawn --system-call-filter=~unshare.)