PATCH for server/dhcp.c

[Ted Lemon]

The reason not to send a DHCPNAK on an unknown lease is that it prevents 
non-cooperating DHCP servers from coexisting.   You really don't want one 
DHCP server to assign an IP address not known to the other server, and then 
have the other server send a DHCPNAK.   So even if a server is authoritative, 
it still won't send a DHCPNAK for an unknown address - only for an address 
that is known to be invalid on the link or that it owns and has not assigned 
to the client that's claiming it.