dhcp server - some patches

[EvilEzh]

>  In some cases, the server needs to send an explicit NAK, in order to
> force the client to re-broadcast a discover.

That's true, but i'll choose when send NAK and when not. Or not respond at 
all.
Of course we can make some rfc check (discover->offer; request->ack,nak; and 
so on).

Actualy i'm implementing it with jradius module :). Good that can work with 
option 82.
And another good thing that i've some devices with java api, so i can 
directly control them togeather with dhcp server/radius auth.

I've running it now for more than week in production. Everything now looks 
ok now.

p.s. if I remember right there was some code which cheks for dhcp client 
"MSFT 98" there can also add code for "udhcp *" and before it was check for 
opcode ==3. opcode never will be 3 (1|2).
Maybe this was for some testing and code isn't needed anymore.