[DHCPv4] add the IPv6-Only Preferred Option

[evverx]

https://datatracker.ietf.org/doc/html/rfc8925#name-option-format

3.1.  Option Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Code      |   Length      |           Value               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Value (cont.)         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 1: IPv6-Only Preferred Option Format

   Fields:

   Code:  8-bit identifier of the IPv6-Only Preferred option code as
      assigned by IANA: 108.

   Length:  8-bit unsigned integer.  The length of the option, excluding
      the Code and Length Fields.  The server MUST set the length field
      to 4.

   Value:  32-bit unsigned integer.  The number of seconds for which the
      client should disable DHCPv4 (V6ONLY_WAIT configuration variable).

The patch was also cross-checked with Wireshark:

    Option: (108) IPv6-Only Preferred
        Length: 4
        Value: ffffffff

[codecov]

Codecov Report

Merging #4108 (17226d9) into master (c57c02b) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4108      +/-   ##
==========================================
- Coverage   81.98%   81.98%   -0.01%     
==========================================
  Files         328      328              
  Lines       76027    76027              
==========================================
- Hits        62333    62328       -5     
- Misses      13694    13699       +5     

Files Changed	Coverage Δ
scapy/layers/dhcp.py	82.41% <ø> (-1.63%)	⬇️

... and 8 files with indirect coverage changes

[guedou]

Thanks for this PR!