plperl strict mode and associated fixes

The attached patch completes (I hope) the work begun by Michael Fuhr in
an earlier unapplied patch, and makes strict mode work as recently
discussed. I moved the embedded strings out of the calling functions
into global macros to try to make the code a little more readable.

Unfortunately we can't have regression tests for this because it relies
on a custom variable class.

Illustration of use:

andrew=# set plperl.use_strict = 'true';
SET
andrew=# create function foo() returns text language plperlu as $$
$foo=1; return 'foo';$$;
ERROR: creation of Perl function failed: Global symbol "$foo" requires
explicit package name at (eval 1) line 1.
andrew=# set plperl.use_strict = 'false';
SET
andrew=# create function foo() returns text language plperlu as $$
$foo=1; return 'foo';$$;
CREATE FUNCTION

cheers

andrew