Add on_trusted_init and on_untrusted_init to plperl [PATCH]

This is the fourth of the patches to be split out from the former
'plperl feature patch 1'.

Changes in this patch:

- Adds plperl.on_trusted_init and plperl.on_untrusted_init GUCs
    Both are PGC_USERSET.
    SPI functions are not available when the code is run.
    Errors are detected and reported as ereport(ERROR, ...)

- select_perl_context() state management improved
    An error during interpreter initialization will leave
    the state (interp_state etc) unchanged.

- The utf8fix code has been greatly simplified.

Tim.