Add \gdesc psql command.

This command acts somewhat like \g, but instead of executing the query
buffer, it merely prints a description of the columns that the query
result would have.  (Of course, this still requires parsing the query;
if parse analysis fails, you get an error anyway.)  We accomplish this
using an unnamed prepared statement, which should be invisible to psql
users.

Pavel Stehule, reviewed by Fabien Coelho

Discussion: https://postgr.es/m/CAFj8pRBhYVvO34FU=EKb=nAF5t3b++krKt1FneCmR0kuF5m-QA@mail.gmail.com

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

@@ -1949,6 +1949,25 @@ Tue Oct 26 21:40:57 CEST 1999
       </varlistentry>
 
 
+      <varlistentry>
+        <term><literal>\gdesc</literal></term>
+
+        <listitem>
+        <para>
+         Shows the description (that is, the column names and data types)
+         of the result of the current query buffer.  The query is not
+         actually executed; however, if it contains some type of syntax
+         error, that error will be reported in the normal way.
+        </para>
+
+        <para>
+         If the current query buffer is empty, the most recently sent query
+         is described instead.
+        </para>
+        </listitem>
+      </varlistentry>
+
+
       <varlistentry>
         <term><literal>\gexec</literal></term>