allow if_output to be specialised on ethernet interfaces

[David Gwynne]

this makes it nicer to set up a custom output routine on ethernet
interfaces. rather than overwriting it after ether_ifattach is called,
you can set it up with the rest of the callbacks and the ether layer
will respect it.

ok?

Index: if_ethersubr.c
===================================================================
RCS file: /cvs/src/sys/net/if_ethersubr.c,v
retrieving revision 1.254
diff -u -p -r1.254 if_ethersubr.c
--- if_ethersubr.c      11 Dec 2018 01:27:08 -0000      1.254
+++ if_ethersubr.c      11 Dec 2018 23:48:17 -0000
@@ -510,7 +510,8 @@ ether_ifattach(struct ifnet *ifp)
        ifp->if_addrlen = ETHER_ADDR_LEN;
        ifp->if_hdrlen = ETHER_HDR_LEN;
        ifp->if_mtu = ETHERMTU;
-       ifp->if_output = ether_output;
+       if (ifp->if_output == NULL)
+               ifp->if_output = ether_output;
        ifp->if_rtrequest = ether_rtrequest;
 
        if_ih_insert(ifp, ether_input, NULL);