psql: Add command to use extended query protocol

This adds a new psql command \bind that sets query parameters and
causes the next query to be sent using the extended query protocol.
Example:

    SELECT $1, $2 \bind 'foo' 'bar' \g

This may be useful for psql scripting, but one of the main purposes is
also to be able to test various aspects of the extended query protocol
from psql and to write tests more easily.

Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/e8dd1cd5-0e04-3598-0518-a605159fe314@enterprisedb.com

src/bin/psql/help.c

@@ -189,6 +189,7 @@ slashUsage(unsigned short int pager)
 	initPQExpBuffer(&buf);
 
 	HELP0("General\n");
+	HELP0("  \\bind [PARAM]...       set query parameters\n");
 	HELP0("  \\copyright             show PostgreSQL usage and distribution terms\n");
 	HELP0("  \\crosstabview [COLUMNS] execute query and display result in crosstab\n");
 	HELP0("  \\errverbose            show most recent error message at maximum verbosity\n");