Add on_perl_init and proper destruction to plperl UPDATED [PATCH]

This is an updated version of the third of the patches to be split out
from the former 'plperl feature patch 1'.

It includes changes following discussions with Tom Lane and others.

Changes in this patch:

- Added plperl.on_perl_init GUC for DBA use (PGC_SIGHUP)
    SPI functions are not available when the code is run.

- Added interpreter destruction behaviour
    Hooked via on_proc_exit().
    Only has any effect for normal shutdown.
    END blocks, if any, are run then objects are
    destroyed, calling their DESTROY methods, if any.
    SPI functions will die if called at this time.

Tim.