First feature patch for plperl - draft [PATCH]

Building on my earlier plperl refactoring patch, here's a draft of my
first plperl feature patch.

Significant changes in this patch:

- New GUC plperl.on_perl_init='...perl...' for admin use.
- New GUC plperl.on_trusted_init='...perl...' for plperl user use.
- New GUC plperl.on_untrusted_init='...perl...' for plperlu user use.
- END blocks now run at backend exit (fixes bug #5066).
- Stored procedure subs are now given names ($name__$oid).
- More error checking and reporting.
- Warnings no longer have an extra newline in the NOTICE text.
- Various minor optimizations like pre-growing data structures.

I'm working on adding tests and documentation now, meanwhile I'd very
much appreciate any feedback on the patch.

Tim.

p.s. Once this patch is complete I plan to work on patches that:
- add quote_literal and quote_identifier functions in C.
- generalize the Safe setup code to enable more control.
- formalize namespace usage, moving things out of main::
- add a way to perform inter-sub calling (at least for simple cases).
- possibly rewrite _plperl_to_pg_array in C.