psql: Add more information about service name

This commit adds support for the following items in psql, able to show a
service name, when available:
- Variable SERVICE.
- Substitution %s in PROMPT{1,2,3}.

This relies on 4b99fed754, that has made the service name available in
PGconn for libpq.

Author: Michael Banck
Reviewed-by: Greg Sabino Mullane
Discussion: https://postgr.es/m/6723c612.050a0220.1567f4.b94a@mx.google.com

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

@@ -4380,6 +4380,15 @@ bar
         </listitem>
       </varlistentry>
 
+      <varlistentry id="app-psql-variables-service">
+        <term><varname>SERVICE</varname></term>
+        <listitem>
+        <para>
+        The service name, if applicable.
+        </para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry id="app-psql-variables-shell-error">
        <term><varname>SHELL_ERROR</varname></term>
        <listitem>
@@ -4674,6 +4683,11 @@ testdb=&gt; <userinput>INSERT INTO my_table VALUES (:'content');</userinput>
         </listitem>
       </varlistentry>
 
+      <varlistentry id="app-psql-prompting-s">
+        <term><literal>%s</literal></term>
+        <listitem><para>The name of the service.</para></listitem>
+      </varlistentry>
+
       <varlistentry id="app-psql-prompting-slash">
         <term><literal>%/</literal></term>
         <listitem><para>The name of the current database.</para></listitem>