main: relax file ownership checks if running under euid 0
main: relax file ownership checks if running under euid 0

During startup, we check that the data files are owned by our euid.
But in a container environment, this is impossible to enforce because
uid/username mappings are different between the host and the container,
and the data files are likely to be mounted from the host.

To allow for such environments, relax the checks if euid=0. This
both matches what happens in a container (programs run as root) and
the kernel access checks (euid 0 can do anything).

We can reconsider this when container uid mapping is better developed.

Fixes #4823.
Fixes #4536.

avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla