Refactor DHCP implementation to comply with RFC 2131

[acd407]

This commit refactors the DHCP functionality in VPCS to better align with RFC 2131 specifications:

1. Renamed the boolean 'renew' parameter to enum 'stage' in dhcp4_request() to properly distinguish between DHCP phases:

   • stage = 0: Initial DHCP request (INIT)
   • stage = 1: Renewing request (RENEWING)
   • stage = 2: Rebinding request (REBINDING)

2. Corrected IP address handling for each phase:

   • INIT phase (stage = 0): Source IP set to 0.0.0.0
   • RENEWING phase (stage = 1): Source IP set to client's current IP
   • REBINDING phase (stage = 2): Source IP set to client's current IP

3. Updated all dhcp4_request() calls to pass appropriate stage values:

   • run_dhcp_new() selects stage based on user input
   • dhcp_renew() passes stage = 1
   • dhcp_rebind() passes stage = 2

4. Maintained backward compatibility by keeping the 'renew' parameter in dhcp4_discover()

5. Improved code documentation and removed redundant comments

These changes ensure proper DHCP behavior according to RFC 2131 while maintaining compatibility with existing code.

[acd407]

Before this commit, systemd-resolved did not accept DHCP requests from VPCS, like this:

PC1> dhcp -d
Opcode: 1 (REQUEST)
Client IP Address: 0.0.0.0
Your IP Address: 0.0.0.0
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Discover
Option 12: Host Name = PC1
Option 61: Client Identifier = Hardware Type=Ethernet MAC Address = 00:50:79:66:68:00

Opcode: 2 (REPLY)
Client IP Address: 0.0.0.0
Your IP Address: 192.168.16.64
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Offer
Option 51: Lease Time = 3600
Option 1: Subnet Mask = 255.255.255.0
Option 3: Router = 192.168.16.1
Option 6: DNS Server = 192.168.16.1 
Option 54: DHCP Server = 192.168.16.1

Opcode: 1 (REQUEST)
Client IP Address: 192.168.16.64
Your IP Address: 0.0.0.0
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Request
Option 54: DHCP Server = 192.168.16.1
Option 50: Requested IP Address = 192.168.16.64
Option 61: Client Identifier = Hardware Type=Ethernet MAC Address = 00:50:79:66:68:00
Option 12: Host Name = PC1

Opcode: 1 (REQUEST)
Client IP Address: 192.168.16.64
Your IP Address: 0.0.0.0
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Request
Option 54: DHCP Server = 192.168.16.1
Option 50: Requested IP Address = 192.168.16.64
Option 61: Client Identifier = Hardware Type=Ethernet MAC Address = 00:50:79:66:68:00
Option 12: Host Name = PC1

Opcode: 1 (REQUEST)
Client IP Address: 192.168.16.64
Your IP Address: 0.0.0.0
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Request
Option 54: DHCP Server = 192.168.16.1
Option 50: Requested IP Address = 192.168.16.64
Option 61: Client Identifier = Hardware Type=Ethernet MAC Address = 00:50:79:66:68:00
Option 12: Host Name = PC1


Can't get ip address from dhcp server

After this commit, DHCP can proceed normally:

PC1> dhcp -d
Opcode: 1 (REQUEST)
Client IP Address: 0.0.0.0
Your IP Address: 0.0.0.0
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Discover
Option 12: Host Name = PC1
Option 61: Client Identifier = Hardware Type=Ethernet MAC Address = 00:50:79:66:68:00

Opcode: 2 (REPLY)
Client IP Address: 0.0.0.0
Your IP Address: 192.168.16.64
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Offer
Option 51: Lease Time = 3600
Option 1: Subnet Mask = 255.255.255.0
Option 3: Router = 192.168.16.1
Option 6: DNS Server = 192.168.16.1 
Option 54: DHCP Server = 192.168.16.1

Opcode: 1 (REQUEST)
Client IP Address: 0.0.0.0
Your IP Address: 0.0.0.0
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Request
Option 54: DHCP Server = 192.168.16.1
Option 50: Requested IP Address = 192.168.16.64
Option 61: Client Identifier = Hardware Type=Ethernet MAC Address = 00:50:79:66:68:00
Option 12: Host Name = PC1

Opcode: 2 (REPLY)
Client IP Address: 0.0.0.0
Your IP Address: 192.168.16.64
Server IP Address: 0.0.0.0
Gateway IP Address: 0.0.0.0
Client MAC Address: 00:50:79:66:68:00
Option 53: Message Type = Ack
Option 51: Lease Time = 3600
Option 1: Subnet Mask = 255.255.255.0
Option 3: Router = 192.168.16.1
Option 6: DNS Server = 192.168.16.1 
Option 54: DHCP Server = 192.168.16.1

 IP 192.168.16.64/24 GW 192.168.16.1