Rework multicast join

 * Do not force definitions on Windows so that compilation works with
   all variants of MingW (this may break IPv6 SSM with old compilers).
 * Do not fall back to legacy IPv4-specific ioctls if RFC3678 is
   supported. This avoids compiling dead code and works around issues
   with Linux kernel headers (especially Android).
 * Do not attempt to set multicast interface with legacy IPv4 ioctls.
   It is not very clear whether this is the address or the index.
   Also, "--miface-addr" was very confusing as it was mostly ignored in
   favor of "--miface".