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

doc/src/sgml/ref/psql-ref.sgml

@@ -917,6 +917,36 @@ INSERT INTO tbl1 VALUES ($1, $2) \bind 'first value' 'second value' \g
        </listitem>
       </varlistentry>
 
+      <varlistentry id="app-psql-meta-command-bind-named">
+       <term><literal>\bind_named</literal> <replaceable class="parameter">statement_name</replaceable> [ <replaceable class="parameter">parameter</replaceable> ] ... </term>
+
+       <listitem>
+        <para>
+         <literal>\bind_named</literal> is equivalent to <literal>\bind</literal>,
+         except that it takes the name of an existing prepared statement as
+         first parameter. An empty string denotes the unnamed prepared
+         statement.
+        </para>
+
+        <para>
+         Example:
+<programlisting>
+INSERT INTO tbls1 VALUES ($1, $2) \parse stmt1
+\bind_named stmt1 'first value' 'second value' \g
+</programlisting>
+        </para>
+
+        <para>
+         This command causes the extended query protocol (see
+         <xref linkend="protocol-query-concepts"/>) to be used, unlike normal
+         <application>psql</application> operation, which uses the simple
+         query protocol. So this command can be useful to test the extended
+         query protocol from <application>psql</application>.
+        </para>
+
+       </listitem>
+      </varlistentry>
+
       <varlistentry id="app-psql-meta-command-c-lc">
         <term><literal>\c</literal> or <literal>\connect [ -reuse-previous=<replaceable class="parameter">on|off</replaceable> ] [ <replaceable class="parameter">dbname</replaceable> [ <replaceable class="parameter">username</replaceable> ] [ <replaceable class="parameter">host</replaceable> ] [ <replaceable class="parameter">port</replaceable> ] | <replaceable class="parameter">conninfo</replaceable> ]</literal></term>
         <listitem>
@@ -1038,6 +1068,35 @@ INSERT INTO tbl1 VALUES ($1, $2) \bind 'first value' 'second value' \g
         </listitem>
       </varlistentry>
 
+      <varlistentry id="app-psql-meta-command-close">
+       <term><literal>\close</literal> <replaceable class="parameter">prepared_statement_name</replaceable></term>
+
+       <listitem>
+        <para>
+         Closes the specified prepared statement. An empty string denotes the
+         unnamed prepared statement. If no prepared statement exists with this
+         name, the operation is a no-op.
+        </para>
+
+        <para>
+         Example:
+<programlisting>
+SELECT $1 \parse stmt1
+\close stmt1
+</programlisting>
+        </para>
+
+        <para>
+         This command causes the extended query protocol to be used,
+         unlike normal <application>psql</application> operation, which
+         uses the simple query protocol. So this command can be useful
+         to test the extended query protocol from
+         <application>psql</application>.
+        </para>
+
+       </listitem>
+      </varlistentry>
+
       <varlistentry id="app-psql-meta-commands-copy">
         <term><literal>\copy { <replaceable class="parameter">table</replaceable> [ ( <replaceable class="parameter">column_list</replaceable> ) ] }
         <literal>from</literal>
@@ -2780,6 +2839,37 @@ lo_import 152801
         </listitem>
       </varlistentry>
 
+      <varlistentry id="app-psql-meta-command-parse">
+        <term><literal>\parse <replaceable class="parameter">statement_name</replaceable></literal></term>
+        <listitem>
+        <para>
+         Creates a prepared statement from the current query buffer, based on
+         the name of a destination prepared-statement object. An empty string
+         denotes the unnamed prepared statement.
+        </para>
+
+        <para>
+         Example:
+<programlisting>
+SELECT $1 \parse stmt1
+</programlisting>
+        </para>
+
+        <para>
+         This command causes the extended query protocol to be used, unlike
+         normal <application>psql</application> operation, which uses the
+         simple query protocol. A
+         <xref linkend="protocol-message-formats-Parse"/>
+         message will be issued by this command so it can be useful to
+         test the extended query protocol from
+         <application>psql</application>. This command affects only the next
+         query executed; all subsequent queries will use the simple query
+         protocol by default.
+        </para>
+
+        </listitem>
+      </varlistentry>
+
       <varlistentry id="app-psql-meta-command-password">
         <term><literal>\password [ <replaceable class="parameter">username</replaceable> ]</literal></term>
         <listitem>