libpq: Add service name to PGconn and PQservice()

This commit adds one field to PGconn for the database service name (if
any), with PQservice() as routine to retrieve it.  Like the other
routines of this area, NULL is returned as result if the connection is
NULL.

A follow-up patch will make use of this feature to be able to display
the service name in the psql prompt.

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

src/interfaces/libpq/libpq-int.h

@@ -394,6 +394,7 @@ struct pg_conn
 	char	   *fbappname;		/* fallback application name */
 	char	   *dbName;			/* database name */
 	char	   *replication;	/* connect as the replication standby? */
+	char	   *pgservice;		/* Postgres service, if any */
 	char	   *pguser;			/* Postgres username and password, if any */
 	char	   *pgpass;
 	char	   *pgpassfile;		/* path to a file containing password(s) */