info: show disk info even if there's no /etc/fstab file

the g_unix_mount_points_get() function checks for entry mount
points. if there are none, it returns NULL and we end up showing
no information about disks.

if that's the case, this patch calls g_unix_mounts_get() which
checks for mounted filesystems in /etc/mtab.

also we need to update the list of ignored filesystems to
include rootfs, which is always present and is a duplicate of
an actual filesystem mount point.

https://bugzilla.gnome.org/show_bug.cgi?id=704702

panels/info/cc-info-panel.c

@@ -608,6 +608,11 @@ get_primary_disc_info (CcInfoPanel *self)
   GList        *p;
 
   points = g_unix_mount_points_get (NULL);
+
+  /* If we do not have /etc/fstab around, try /etc/mtab */
+  if (points == NULL)
+    points = g_unix_mounts_get (NULL);
+
   for (p = points; p != NULL; p = p->next)
     {
       GUnixMountEntry *mount = p->data;

panels/info/gsd-disk-space-helper.c

@@ -71,6 +71,7 @@ gsd_should_ignore_unix_mount (GUnixMountEntry *mount)
                 "proc",
                 "procfs",
                 "ptyfs",
+                "rootfs",
                 "rpc_pipefs",
                 "selinuxfs",
                 "smbfs",