trash: do not care about mount points without read access

The g_file_monitor_directory polls the directory every 2 seconds if
we try to monitor mount points without read access. It prevents autofs
mounts from being expired and causes high system load. Don't watch
mount points without read access at all to fix the problem.

Based on the patch by Scott Mayhew.

https://bugzilla.gnome.org/show_bug.cgi?id=737371