files: Add fdinfo_list_entry::stage

Add fle open stages. Set a stage after every operation.

v2: Do not merge filetype specific state with generic.

travis-ci: success for Rework file opening scheme to make it asynchronous (rev5)
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>