Signalize that CUPS connection is available when EISCONN occurs

Add test for EISCONN error when testing whether a connection to CUPS server
is available (#576678 - patch by mark@ecs.vuw.ac.nz).

Signed-off-by: Marek Kasik <mkasik@redhat.com>

modules/printbackends/cups/gtkcupsutils.c

@@ -1290,7 +1290,7 @@ gtk_cups_connection_test_get_state (GtkCupsConnectionTest *test)
 
           error_code = errno;
 
-          if (code == 0)
+          if (code == 0 || error_code == EISCONN)
             {
               close (test->socket);
               test->socket = -1;