tmpfiles: automatically create /etc/credstore/ and friends

This adds a tmpfiles.d/ snippet for LoadCredential= style credentials
directories in /etc/ and /run/.

This is done primarily to ensure that the access modes for the dirs are
set up properly, in the most restrictive ways. Specifically these are
set to 0000, so that CAP_DAC_OVERRIDE is necessary to enumerate and read
the credentials, and being UID=0 is not sufficient to do so.

This creates /etc/credstore/, but leaves /run/credstore/ absent if
missing, for now. Thinking is: the latter being non-persistent is
created by software usually, not manually by users, and hence more
likely right. But dunno, we might want to revisit this sooner or later.

This is ultimately an exercise to advertise the LoadCredential= concept
a bit, and do so in a reasonably secure way, underlining the safety of
the concept.

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