Integerize mg.c; eliminate warning on C< local($)) >

Integerize mg.c; eliminate warning on C< local($)) >

Ok, here's a tested patch for the debugger problem.
I was missing the fact that DB::dbline magic is actually
uppercase (which means hv_store_ent() etc., will pass SV keys
to the vtbl_dbline handlers).

Replace the oft-repeated mg_ptr incantation with
the simple MgPVKEY macro.

Rename MgPVKEY to MgPV (to match with HePV elsewhere).  Add
additional parens around the "mg".

(lines near 584) Part of VMS changes.  I don't know what this did.

This patch converts magic variables ($!, $^E, etc.) to use integers
(C<sv_setiv>) instead of floats.  It also eliminates a warning from
C< local($)) >, via a hack similar to $!.