Changelist:
- Replace ND, D and RD macros with nm_prdis, nm_prinf, nm_prerr
and nm_prlim, to avoid possible naming conflicts.
- Add netmap_krings_mode_commit() helper function and use that
to reduce code duplication.
- Refactor pipes control code to export some functions that
can be reused by the veth driver (on Linux) and epair(4).
- Add check to reject API requests with version less than 11.
- Small code refactoring for the null adapter.
MFC after: 1 week
75f4f3ed51