pg_upgrade: revert changing '' to ""

On the command line, GUC option strings are handled by the guc parser,
not by the shell parser, so '' is the proper way to represent a
zero-length string.  This reverts commit
3132a9b7ab.

contrib/pg_upgrade/server.c

@@ -190,7 +190,7 @@ start_postmaster(ClusterInfo *cluster, bool throw_error)
 #ifdef HAVE_UNIX_SOCKETS
 	/* prevent TCP/IP connections, restrict socket access */
 	strcat(socket_string,
-		   " -c listen_addresses=\"\" -c unix_socket_permissions=0700");
+		   " -c listen_addresses='' -c unix_socket_permissions=0700");
 
 	/* Have a sockdir?	Tell the postmaster. */
 	if (cluster->sockdir)