Add a new system view, pg_prepared_statements, that can be used to

access information about the prepared statements that are available
in the current session. Original patch from Joachim Wieland, various
improvements by Neil Conway.

The "statement" column of the view contains the literal query string
sent by the client, without any rewriting or pretty printing. This
means that prepared statements created via SQL will be prefixed with
"PREPARE ... AS ", whereas those prepared via the FE/BE protocol will
not. That is unfortunate, but discussion on -patches did not yield an
efficient way to improve this, and there is some merit in returning
exactly what the client sent to the backend.

Catalog version bumped, regression tests updated.