Add an nvme_function structure array, defining the name, C function and

Add an nvme_function structure array, defining the name, C function and
usage message for each nvmecontrol command.  This helps reduce some code
clutter both now and for future commits which will add logpage and
firmware support to nvmecontrol(8).

Also move helper function prototypes to the end of the header file, after
the per-command functions.

Sponsored by:	Intel
MFC after:	3 days