If we have ether_hostton(), check whether we have <netinet/if_ether.h>
and if including it declares ether_hostton(), and define
HAVE_DECL_ETHER_HOSTTON appropriately, and use that to determine whether
to define it ourselves, rather than having a set of OSes that don't need
it, as that set can change over time.

Make the default declaration of "ether_hostton()" declare its first
argument as "const char *", as that's what it's *supposed* to be
(although it's not declared as such in some OSes, so we still have to
cast a "const char *" when passed as the first argument).