machined: in --user mode, restrict register access to our own UID, and that's it

machined: in --user mode, restrict register access to our own UID, and that's it

This is a follow-up for 119d332d9c2cf1974b235c8d9e4e3ad821cf436a and
ensures the check only is applied to the system instance of machined. It
doesn't really apply to the per-user instance, because we never want to
permit differently privileged clients access anyway.

(The process_is_owned_by_uid() call might fail if invoked unpriv, hence
there's value in not calling it if machined runs in --user mode, it's
what makes machined actually work)