lib: update Solaris multicast API (BZ#725)

lib: update Solaris multicast API (BZ#725)

On OpenIndiana/Solaris the build fails with "unsupported multicast API".
It's only in the IPv4 part where setsockopt IP_MULTICAST_IF needs a
local address and not the index (IPv6 wants the index).
The following code walks the list of interfaces until it finds the matching
index and uses the interface's local address for the setsockopt call.
I don't know if it works on Solaris < 10 (I guess yes, but I don't have
any machine to verify it).

[NB: this breaks unnumbered setups that use the same IPv4 address on
multiple interfaces. -- equinox@opensourcerouting.org]

Reported-by: Brian Utterback <brian.utterback@oracle.com>
Signed-off-by: Christian Franke <chris@opensourcerouting.org>
Patchwork #762