OpenBSD: Don't try to read System process arguments

Don't try to get the process args for system processes.

According to the OpenBSD source here they have No sigs, stats or swapping.

Trying to get the process args anyway just fails and causes needless log spam.

This happens, for example, when displaying My Processes in a gnome-system-monitor as root.

Alternatively, if failing silently isn't expected behavior, I would be open to adding a separate, more descriptive, warning to this special case.