raw.c, udp.c, init.c, opt.h, ip.h, sockets.h: bug #26309: Implement the SO(F)_BROADCA...

raw.c, udp.c, init.c, opt.h, ip.h, sockets.h: bug #26309: Implement the SO(F)_BROADCAST filter for all API layers. Avoid the unindented reception of broadcast packets even when this option wasn't set. Port maintainers which want to enable this filter have to set IP_SOF_BROADCAST=1 in opt.h. If you want this option also filter broadcast on recv operations, you also have to set IP_SOF_BROADCAST_RECV=1 in opt.h.