Create package (or include in a related package) support for detecting whether a requested TCP port is officially reserved?

[atc0005]

Context from a GH issue I just updated:

Based on the current validation logic (and why it is applying the checks that it is), the user should try to pick an unused (and preferably unreserved) port between 1024 and 49151 for the best results.

If we had an easy way to determine whether a requested port was in that range via quick lookup we could emit a warning about that too.

More context:

• https://github.com/atc0005/brick/blob/e7e10fc430e9f78de8a9c6b1cebfbe71a2da778c/config/validate.go#L33-L78
• atc0005/brick#41