fam: implement gio-nfs-{file,directory}-monitor

Declare explicit support for monitor NFS from the fam file monitoring
backend.  This will cause it to be preferred for monitoring on NFS, if
it is installed.

https://bugzilla.gnome.org/show_bug.cgi?id=592211

gio/fam/gfamfilemonitor.c

@@ -147,5 +147,9 @@ g_fam_file_monitor_register (GIOModule *module)
 				  G_TYPE_FAM_FILE_MONITOR,
 				  "fam",
 				  10);
+  g_io_extension_point_implement (G_NFS_FILE_MONITOR_EXTENSION_POINT_NAME,
+				  G_TYPE_FAM_FILE_MONITOR,
+				  "fam",
+				  10);
 }