assoc some more special SV syms, fixes #306 $/

  $\ => PL_ors_sv
  $/ => PL_rs
  $@ => PL_errors

This list might not be complete yet and I'm unsure if local $/ to other
values than undef will work.
Added local $/ testcase to t/issue256.t.
See #256 for tracking all such special symbols.