Use syslog for logging

/etc/nvidia-container-runtime/config.toml allows system-wide specification of a debug file, with the default being "/var/log/nvidia-container-toolkit.log"

Using /var/log is a blocker for rootless usage. Either the system-wide configuration has to be changed to a world-writable path or logging needs to be disabled.

This issue proposes that syslog be used instead. Which permits a system administrator to configure and direct logs for user containers to locations appropriate for each user.

(this was discovered during integration with podman.)