Create a local copy of the PMC if necessary. (No copy is made if it
is marked shared.) This includes workarounds for Parrot_clone() not
doing the Right Thing with subroutines (specifically, code segments
aren't preserved and it is difficult to do so as long as
Parrot_clone() depends on freezing).

##</code><code>##

Fixup a PMC to be sharable. Right now, reassigns the vtable to one
owned by some master interpreter, so the PMC can be safely reused
after thread death.

In the future the PMC returned might be different than the one
passed, e.g., if we need to reallocate the PMC in a different
interpreter.