Fix/Update Nmap Continuous Integration (CI) system

[fyodor]

Nmap previously used Travis for continuous integration (CI). That stopped working some years ago (possibly related to #1494) and we haven't fixed Travis or migrated to a new system yet. But we probably should since it would be very valuable to have Nmap regularly built and tested on many platforms. Nmap does have a "make check" Makefile rule, and having a working CI again might inspire us to expand the testing framework.

[fyodor]

Also note that there was a PR #2564 for replacing Travis with GitHub actions that we could consider.

[fyodor]

Also worth noting that for Npcap we currently use Appveyor: https://ci.appveyor.com/project/dmiller-nmap/npcap

It would be convenient to use the same platform for both Nmap and Npcap if practicle.

[Ansuel]

Made all the heavy work here #2826