image: check if image owner == root and perms == CR_FD_PERM

Currently, criu accepts any images. Lets check if owner == root and
perms == CR_FD_PERM to ensure that image wasn't modified by unprivileged
user. If it isn't so, print warning and check who is currently using criu.
If user != root, don't allow to use this image and return error. Root, in
his turn, can use any images.