errcontext support in PL/Perl

Hi,

While trying to build a custom error reporting function for one of our
clients we came across the fact that PL/Perl doesn't set errcontext
that we relied on to get the traceback of running functions. Exactly
the same problem with PL/Python was fixed recently by Peter Eisentraut
(http://archives.postgresql.org/pgsql-committers/2009-07/msg00168.php).

Attached is a patch (HEAD) that sets errcontext with PL/Perl function
name, making a distinction between compilation and execution stages,
fixes error messages where function name was already included in the
message itself and updates regression tests. I'll appreciate any
suggestions on how to improve it.