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>

src/bin/psql/command.c

@@ -4082,6 +4082,7 @@ SyncVariables(void)
 	pset.sversion = PQserverVersion(pset.db);
 
 	SetVariable(pset.vars, "DBNAME", PQdb(pset.db));
+	SetVariable(pset.vars, "SERVICE", PQservice(pset.db));
 	SetVariable(pset.vars, "USER", PQuser(pset.db));
 	SetVariable(pset.vars, "HOST", PQhost(pset.db));
 	SetVariable(pset.vars, "PORT", PQport(pset.db));
@@ -4115,6 +4116,7 @@ void
 UnsyncVariables(void)
 {
 	SetVariable(pset.vars, "DBNAME", NULL);
+	SetVariable(pset.vars, "SERVICE", NULL);
 	SetVariable(pset.vars, "USER", NULL);
 	SetVariable(pset.vars, "HOST", NULL);
 	SetVariable(pset.vars, "PORT", NULL);

src/bin/psql/prompt.c

@@ -33,6 +33,7 @@
  * %p - backend pid
  * %> - database server port number
  * %n - database user name
+ * %s - service
  * %/ - current database
  * %~ - like %/ but "~" when database name equals user name
  * %w - whitespace of the same width as the most recent output of PROMPT1
@@ -165,6 +166,11 @@ get_prompt(promptStatus_t status, ConditionalStack cstack)
 					if (pset.db)
 						strlcpy(buf, session_username(), sizeof(buf));
 					break;
+					/* service name */
+				case 's':
+					if (pset.db && PQservice(pset.db))
+						strlcpy(buf, PQservice(pset.db), sizeof(buf));
+					break;
 					/* backend pid */
 				case 'p':
 					if (pset.db)