WIP 2 interpreters for plperl

I have made some progress with what I think is needed to have two
interpreters for plperl. This is a lot harder than the pltcl case for
two reasons: 1. there are no restrictions on having 2 tcl interpreters,
and 2. tcl does not need to save and restore context as we have to do
with perl. I think I have a conceptual siolution to these two problems,
but what I have is currently segfaulting somewhat myteriously. Tracing a
dynamically loaded library in a postgres backend with a debugger is less
than fun, too. I am attaching what I currently have, liberally sprinkled
with elog(NOTICE) calls as trace writes.

I need to get some other work done today too, so I will return to this
later if I can. In the meanwhile, if anybody cares to cast a fresh set
of eyeballs over this, please do.

cheers

andrew