kern - Add kether_ntoa, kether_aton functions.

- kether_aton() converts a human readable MAC string to an
  ethernet address.
- kether_ntoa() converts ethernet addresses into human readable
  MAC string.

Human readable notation is specified in IEEE 802.
kern - Utilize new way of printing MAC addresses.

kether_aton() function is being used to return a buffer
with the human readable notation of an ethernet address.
libkern - Add new function hexncpy.
kern - More work replacing %D

Use hexncpy() for special cases where, for example,
a different separator might be needed.
Remove %D conversion from kprintf(9) and the GCCs.

It has been removed also from libstand's printf.

While useful, these non-standard conversions have the downside that
each compiler needs adjusting to support proper printf format warnings
for them.

We have now a kether_ntoa() function to serve the purpose of %D