Indicate why we have incomplete definitions for "struct mbuf" and

Indicate why we have incomplete definitions for "struct mbuf" and
"struct rtentry" (in <net/if.h> on some systems, e.g.  Digital UNIX 4.0,
there are prototypes for kernel functions that include "struct mbuf *"
and "struct rtentry *" arguments, and they are included even if you're
just user-mode code).

We don't need to include <netinet/if_ether.h> unless we have
"ether_hostton()", and we don't need to include <net/if.h> unless we're
including <netinet/if_ether.h>, and we don't need to define "struct
mbuf" or "struct rtentry" unless we're including <net/if.h>.