Make Kavorka thread-safe

Devel::Pragma turns out not to be thread safe on 64 bit
systems. However, since we're only using it for a very simple use case,
we can replace it by implementing our own Kavorka::fqname function and
having Kavorka::_exporter_validate_opts use %^H to stash the equivalent
of Devel::Pragma::ccstash for retrieval by our fqname implementation.

Job done. Thread safe. Pure Perl.