netmap: refactor logging macros and pipes

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