Add on_plperl_init and on_plperlu_init to plperl UPDATE 3 [PATCH]

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

Changes in this patch:

- Added plperl.on_plperl_init and plperl.on_plperlu_init GUCs
    Both are PGC_SUSET
    SPI functions are not available when the code is run.
    Errors are detected and reported as ereport(ERROR, ...)
    Corresponding documentation and tests for both.

- Renamed plperl.on_perl_init to plperl.on_init

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

- The utf8fix code has been greatly simplified.

- More code comments re PGC_SUSET and no access to SPI functions.

Tim.