Add DHCP server support including support for UDP server in uIP

[logicog]

This adds support for a DHCP server. The code reuses the DHCP client implementation as it does not make sense to run a DHCP server at the same time as a client. The structures and state machine can therefore be re-used to play the other party in the DHCP handshake.

Enable DHCPD on the CLI:

> ip
Current IP: 192.168.10.247 (static)

> dhcpd on
dhcpd_start done

> parse_dhcp_request called
dhcp_send_reply called
parse_dhcp_request called
dhcp_send_reply called
parse_dhcp_request called
Client name: DesktopPC
PARAMS request (ignored)
dhcp_send_reply called

And run requests on a connected Linux machine using one of the many dhcp client implementations:

$ sudo dhclient -v enx00e04c000001
Internet Systems Consortium DHCP Client 4.4.3-P1
Copyright 2004-2022 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/enx00e04c000001/00:e0:4c:00:00:01
Sending on   LPF/enx00e04c000001/00:e0:4c:00:00:01
Sending on   Socket/fallback
DHCPREQUEST for 192.168.10.100 on enx00e04c000001 to 255.255.255.255 port 67
DHCPREQUEST for 192.168.10.100 on enx00e04c000001 to 255.255.255.255 port 67
DHCPDISCOVER on enx00e04c000001 to 255.255.255.255 port 67 interval 7
DHCPOFFER of 192.168.10.100 from 192.168.10.247
DHCPREQUEST for 192.168.10.100 on enx00e04c000001 to 255.255.255.255 port 67
DHCPACK of 192.168.10.100 from 192.168.10.247
bound to 192.168.10.100 -- renewal in 20483 seconds.


$ sudo /usr/sbin/dhcpcd -4 enx00e04c000001
dhcpcd-10.1.0 starting
DUID 00:01:00:01:30:ff:bb:c1:00:e0:4c:00:00:01
enx00e04c000001: IAID 4c:00:00:01
enx00e04c000001: soliciting a DHCP lease
enx00e04c000001: offered 192.168.10.100 from 192.168.10.247
enx00e04c000001: probing for an IPv4LL address
enx00e04c000001: using IPv4LL address 169.254.65.105
enx00e04c000001: adding route to 169.254.0.0/16
enx00e04c000001: adding default route

The server is very simple and at present not very configurable. It will hand out up to 20 IP addresses in the range xx.yy.zz.100 - xx.yy.zz.120 where xx.yy.zz.ii is the IP address of the switch.