image: Shrink signal entry structure

Since we use pure syscalls there is no
need to keep intermediate layer for signals.

Moreover mask entry moved at the end of the structure
so we will easily expand it if it'll be ever needed.

Note it breaks backward compatibility with older image
but since it's development stage it should be safe.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>