[MERGE] Improve -Dx, -Dt, esp in threaded builds

This series of commits improves the way various OPs are displayed by
Perl_op_dump() (as used by perl -Dx) and by Perl_debop() (as used by
perl -Dt).

In particular it tries, where possible, to display information about an
SV or GV attached to the OP when that SV has been moved to the pad on
threaded builds.

Author: iabyn