These are the mods necessary to 4.3BSD to add a subnet ARP hack.
They allow a subnet gateway machine to act as a bridge so that only
it need know of the existence of subnets.  This is quite useful if
many machines on the local network do not have RFC950 subnet code
and you do not have sources for them.

To turn on the subarp code, you need to do "ifconfig xxn subarp".

/sys/netinet/if_ether.c:	subnet ARP hack
/sys/net/if.h:			IFF_SUBARP
/sys/net/route.c:		splimp instead of splnet in rtalloc
/sys/netinet/in.c:		in_mainnetof
/sys/netinet/in.h:		in_mainnetof
/usr/src/etc/ifconfig/ifconfig.c: IFF_SUBARP (subarp, -subarp)
/usr/man/man8/ifconfig.8c:	subarp, -subarp

