Using DLPI with HP-UX 11

[Johannes Berg]

> http://johannes.sipsolutions.net/wiki/Projects/HP-DLPI-DHCP

Doing this surfaced a bug in dhcpd -- you can't define USE_SOCKET_RECEIVE
without USE_SOCKET_SEND because then the fallback interface will screw you
up. The solution is to add a global variable in common/socket.c to store
the rfdesc and use that in the fallback logic instead of interface ->
rfdesc (which points to fallback_interface where rfdesc is still -1)

Additionally, the cited code contains another bug in send_packet which
I've fixed locally, but haven't uploaded the fix yet.

johannes