Introduce UNKNOWN license

[AMDmi3]

Summary

Some ports do not define license. These may be unsafe to use from legal point of view, so we need to state that in the port's Makefiles. Also we need to distinguish ports with unspecified license from ports which have not yet set LICENSE.

So introduce UNKNOWN license.

Topics for dicussion:

• Name: UNSPECIFIED/NONE/UNKNOWN/lowercase equivalents?
• Default permissions: auto-accept to preserve current bahavior and not bug users with a need to confirm the license or no-auto-accept to be on a safe side?

We may instead consider a special handling for these cases, e.g. using NO_LICENSE knob instead, with separate warnings and a way to allow/disallow ports marked as such. This would be non-generic are require more code though.

Test Plan

This license is used for net/p5-Net-Ping, which has unknown license (as explicitly stated in META.json and META.yml)

[AMDmi3]

As suggested by bapt@ and to avoid confusion, rename to NONE (e.g. no license specified)