Summary

Allow and use frames carrying FOpts with no FPort
Close #2455

Changes

• For frames carrying only MAC commands, Network Server now attempts to fit them in FOpts omitting FPort, if possible, and sends them in FRMPayload with FPort 0 as usual otherwise.
• Network Server now properly handles FPort 0 data uplinks carrying FOpts.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md. The target branch is set to master if the changes are fully compatible with existing API, database, configuration and CLI.
• Testing: The changes are covered with unit tests. The changes are tested manually as well.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.