Checkpoint and restore of fifo is similar to
pipes c/r except the pipe end-points are named
file.

Because the fifo has a name we use regular files
facility for fifo path c/r.

Still there is a trick used to "open" fifo:
the opening procedure migh sleep if a fifo's peer
is not yet opened, so before doing a real open
we yield a fake open procedure (with O_RDWR flag)
which prevents us from sleeping even if peer
is not yet ready. Also we need writable fifo
end to restore data queued.

v2:
 - add open/priv members to reg_file_info
 - make open_fifo_fd to use open_fe_fd
 - comment on pipe_id
 - make sure the fifo data is not restored twice

v3:
 - drop useless fixme comment and add sane one
v4:
 - Use restore_data flag to escape data restore duplication
 - Use S_ISREG for file contents copying
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>