FreeBSD's is presently 46 bytes. CID 1422869 warns that it can get overrun
in if_attach() in copying if_xname which is IF_NAMESIZE bytes (32).
This breaks ABI, but BeOS did not have sockaddr_dl, it is only a modern-GCC
ABI break. Since most applications assume that sockaddr_dl is variable-length
and is null-terminated, as well as not used very often, hopefully this will
require relatively few rebuilds.