tmpfiles: when /etc is not fully initialized, some specifiers are expected to be...

tmpfiles: when /etc is not fully initialized, some specifiers are expected to be unresolvable (#6860)

In chroot environments, /etc might not be fully initialized: /etc/machine-id
can be missing for example. This makes the expansions of affected specifiers
impossible at that time.

These cases should not be considered as errors and such failures shouldn't be
logged at an error level therefore this patch downgrades the level used to
LOG_NOTICE in such cases.

Also this is logged at LOG_NOTICE only the first time and then downgrade to
LOG_DEBUG for the rest. That way, if debugging is enabled we get the full
output, but otherwise we only see only one message.

The expansion of specifiers is now self contained in a dedicated function
instead of being spread all over the place.