Currently, libcriu is connecting to CRIU service
by itself, just asking user for a path to socket.
But in some cases users need to provide fd instead
path. For example, sometimes task has no access to
criu socket because of strict security mesures, but
is able to inherit fd from a parent that has access
to criu socket.

v2, use union for addr and fd
Signed-off-by: Ruslan Kuprieiev <rkuprieiev@cloudlinux.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>