Changes

core: always build osmo_sock_multiaddr_* helpers

Always build the public osmo_sock_multiaddr_* helper functions,
independent of HAVE_LIBSCTP.

Move the HAVE_LIBSCTP checks inside the functions and limit them
to the SCTP-specific code paths: when IPPROTO_SCTP is requested
without libsctp support, return -ENOTSUP, while non-SCTP protocols
continue to work as before.

This fixes link failures in libosmo-netif when building with
--disable-libsctp.

Change-Id: I3e70b7cd6cb4d022252e6ddc70a42ca5eea72bb1