Returns an array of fields that each
ProcTableStruct will contain. This may be useful if you want to know in
advance what fields are available
without having to perform at least one read of the /proc table.
In block form, yields a ProcTableStruct for each process entry that you
have rights to. This method returns an array of ProcTableStruct‘s in
non-block form.
If a pid is provided, then only a single ProcTableStruct is
yielded or returned, or nil if no process information is found for that
pid.