=for apidoc mg_localize
Copy some of the magic from an existing SV to new localized version of that
-SV. Container magic (eg C<%ENV>, C<$1>, C<tie>)
-gets copied, value magic doesn't (eg
+SV. Container magic (I<e.g.>, C<%ENV>, C<$1>, C<tie>)
+gets copied, value magic doesn't (I<e.g.>,
C<taint>, C<pos>).
If C<setmagic> is false then no set magic will be called on the new (empty) SV.