[PATCH] Add additional extended protocol commands to psql: \parse and \bindx

Hi all!

Currently, only unnamed prepared statements are supported by psql with the
\bind command and it's not possible to create or use named prepared statements
through extended protocol.

This patch introduces 2 additional commands: \parse and \bindx.
\parse allows us to issue a Parse message to create a named prepared statement
through extended protocol.
\bindx allows to bind and execute a named prepared statement through extended
protocol.

The main goal is to provide more ways to test extended protocol in
regression tests
similarly to what \bind is doing.

Regards,
Anthonin