lib: fix setting of IPv4 multicast sockopts on OpenBSD

lib: fix setting of IPv4 multicast sockopts on OpenBSD

OpenBSD doesn't support the "ifindex hack" derived from RFC 1724 which
allows an ifindex to be encoded in the imr_interface field (in_addr)
of the ip_mreq structure. OpenBSD also doesn't support the RFC3678
Protocol-Independent socket API extensions, which allows an interface
to be specified by its ifindex. With that said, in OpenBSD we still need
to specify an interface by its IP address. This patch adds an exception
in the multicast sockopt functions to handle this case.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>