udp: fix error messages for IPv6 multicast not supported
webui: load mpegts.js even if linuxdvb is disabled

DVB input tabs are also needed for IPTV service.
Also this fixes webui which doesn't load properly
when --disable-linuxdvb is used.
udp: use if_nametoindex instead of SIOCGIFINDEX ioctl

if_nametoindex is more portable and shorter
udp: some compilers doesn't like return = syntax