The only user is fifo, but it uses collect_fd()
to do read only actions (the only change is
fifo_info::reg_d population, which may be easily
moved to open_fifo_fd()).

So, move the collect_special_file() functionality
from collect_fifo_fd() to open_fifo_fd() and
kill collect_fd method at all.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>