image: Add O_OPT when trying to open optional image files

During the time some files become obsolete and might be missing
in checkpoint image set, but to keep backward compatibility we
still trying to open them, which might print out error like

 | Unable to open 'path-to-file'

and confuse a reader why criu prints error but continue working.

To eliminate this problem O_OPT flag has been introduced in
commit 16b5692061e2, which suppress error message priting
if the flag is set.

Now start using O_OPT in the following functions

 - open_irmap_cache: irmap cache is relatively new optional feature

 - prepare_rlimits, open_signal_image, restore_file_locks,
   prepare_fd_pid, prepare_mm_pid, collect_image: all these
   helpers are trying to open image files which can be missing.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>