Fix Virtual DHCP Server: Correct IP reassignment

[hiura2023]

Changes proposed in this pull request:

• DHCP_NAK does not work correctlly

• Changes make it work correctlly

• Description of changes:
  ①If IP address in user's note is changed, then reply to DHCP REQUEST with DHCP NAK.
  This makes DHCP sequence begin with DHCP DISCOVER.
  ②If there is any entry with the same MAC address, then remove it.
  Unused lease time record is deleted from IP lease table.
  ③Reassigning static IP address is partially modified.
  ④Thease changes enable to switch between static and dynamic IP address assignment.
  Users do not need to restart the VPN server.

• Example below:
  Already set to IPv4:192.168.30.222 in user's note.
  First, set to IPv4:192.168.30.234 in user's note. Indicated with yellow line.
  Second, delete IPv4:192.168.30.234 from user's note. Indicated with yellow line.
  Third, set to IPv4:192.168.30.222 in user's note. Indicated with yellow line.
  User can switch the way of IP address assignment at any time.

• Attach A:

Lease Limit:40 seconds
DisableSessionReconnect: true
Time-out Period: 20 seconds

[chipitsine]

this looks like a followup of #1947

@PizzaProgram , can you please test it ?

[chipitsine]

@hiura2023 thank you very much for taking care of OpenSSL-3 issue.

I'm going to merge this PR (I would avoid nitpicking that it should be better to go into 2 separate PRs, anyway, there'll be separate commits, I am fine with that)