Tidy the implementation of Perl_mg_dup().

Tidy the implementation of Perl_mg_dup().

As all the structure elements are directly assigned to, use Newx() rather than
Newxz(). Replace the explicit assignments with a direct structure copy. When
reading values from the parent, read from those copied to the new structure,
rather than the identical value in the old structure, to recduce CPU cache
pressure.