namespace: allow ProcSubset=pid with some ProtectKernel options

namespace: allow ProcSubset=pid with some ProtectKernel options

In case `/proc` is successfully mounted with pid tree subset only due to
`ProcSubset=pid`, the protective mounts for `ProtectKernelTunables=yes` and
`ProtectKernelLogs=yes` to non-pid `/proc` paths are failing because the paths
don't exist. But the pid only option may have failed gracefully (for example
because of ancient kernel), so let's try the mounts but it's not fatal if they
don't succeed.