PR14572: Set s.privilege = unprivileged for stapdyn

PR14572: Set s.privilege = unprivileged for stapdyn

When running under Dyninst, we are effectively unprivileged by nature,
so setting s.privilege to reflect that helps restrict the available
probe types.

However, we still want to allow guru mode for setting target variables
and using embedded-C, so let systemtapr_:session::is_usermodea() pass.

* session.cxx (systemtap_session::parse_cmdline): For --runtime=dyninst,
  set the privilege level too.
  (systemtap_session::check_options): Allow -g for is_usermode().
* staptree.cxx (varuse_collecting_visitor::visit_embeddedcode): Allow
  embedded-C unrestricted for is_usermode().
  (varuse_collecting_visitor::visit_embedded_expr): Ditto.