psql: Add more meta-commands able to use the extended protocol

Currently, only unnamed prepared statement are supported by psql with
the meta-command \bind.  With only this command, it is not possible to
test named statement creation, execution or close through the extended
protocol.

This commit introduces three additional commands:
* \parse creates a prepared statement using the extended protocol,
acting as a wrapper of libpq's PQsendPrepare().
* \bind_named binds and executes an existing prepared statement using
the extended protocol, for PQsendQueryPrepared().
* \close closes an existing prepared statement using the extended
protocol, for PQsendClosePrepared().

This is going to be useful to add regression tests for the extended
query protocol, and I have some plans for that on separate threads.
Note that \bind relies on PQsendQueryParams().

The code of psql is refactored so as bind_flag is replaced by an enum in
_psqlSettings that tracks the type of libpq routine to execute, based on
the meta-command involved, with the default being PQsendQuery().  This
refactoring piece has been written by me, while Anthonin has implemented
the rest.

Author: Anthonin Bonnefoy, Michael Paquier
Reviewed-by: Aleksander Alekseev, Jelte Fennema-Nio
Discussion: https://postgr.es/m/CAO6_XqpSq0Q0kQcVLCbtagY94V2GxNP3zCnR6WnOM8WqXPK4nw@mail.gmail.com

src/test/regress/sql/psql.sql

@@ -45,8 +45,28 @@ SELECT 1 as one, 2 as two \g (format=csv csv_fieldsep='\t')
 SELECT 1 as one, 2 as two \gx (title='foo bar')
 \g
 
--- \bind (extended query protocol)
+-- \parse (extended query protocol)
+\parse
+SELECT 1 \parse ''
+SELECT 2 \parse stmt1
+SELECT $1 \parse stmt2
+SELECT $1, $2 \parse stmt3
 
+-- \bind_named (extended query protocol)
+\bind_named
+\bind_named '' \g
+\bind_named stmt1 \g
+\bind_named stmt2 'foo' \g
+\bind_named stmt3 'foo' 'bar' \g
+
+-- \close (extended query protocol)
+\close
+\close ''
+\close stmt2
+\close stmt2
+SELECT name, statement FROM pg_prepared_statements ORDER BY name;
+
+-- \bind (extended query protocol)
 SELECT 1 \bind \g
 SELECT $1 \bind 'foo' \g
 SELECT $1, $2 \bind 'foo' 'bar' \g
@@ -58,6 +78,9 @@ SELECT foo \bind \g
 SELECT 1 \; SELECT 2 \bind \g
 -- bind error
 SELECT $1, $2 \bind 'foo' \g
+-- bind_named error
+\bind_named stmt2 'baz' \g
+\bind_named stmt3 'baz' \g
 
 -- \gset
 
@@ -990,9 +1013,11 @@ select \if false \\ (bogus \else \\ 42 \endif \\ forty_two;
 	\pset fieldsep | `nosuchcommand` :foo :'foo' :"foo"
 	\a
 	SELECT $1 \bind 1 \g
+	\bind_named stmt1 1 2 \g
 	\C arg1
 	\c arg1 arg2 arg3 arg4
 	\cd arg1
+	\close stmt1
 	\conninfo
 	\copy arg1 arg2 arg3 arg4 arg5 arg6
 	\copyright
@@ -1020,6 +1045,7 @@ select \if false \\ (bogus \else \\ 42 \endif \\ forty_two;
 	\lo_list
 	\o arg1
 	\p
+	SELECT 1 \parse
 	\password arg1
 	\prompt arg1 arg2
 	\pset arg1 arg2