Add SRV record support for win32

[nifanfa]

just for hidding the port. like i can connect to n2n server by using this

edge -c cf -l _n2n_udp.example.com --no-port-forwarding -r -D -E

instead of

edge -c cf -l example.com:7777 --no-port-forwarding -r -D -E

but i don't know how to use a cross-platfrom way to get SRV records. so only win32 is implemented.

Please sign (check) the below before submitting the Pull Request:

• [x] I have signed the ntop Contributor License Agreement at https://github.com/ntop/legal/blob/main/individual-contributor-licence-agreement.md
• [x] I have updated the documentation (in doc/) to reflect the changes made (if applicable)

Link to the related issue:

Describe changes:

[hamishcoleman]

The project you have raised a PR against is no longer very active. If you would like to raise a feature request ticket in the recently released fork n3n, we could discuss things further as part of that project.