Create generic command / arg parsing routines

Create a set of routines and structures to hold the data for the args
for a command. Use them to generate help and to parse args. Convert
all the current commands over to the new format. "comnd" is a hat-tip
to the TOPS-20 %COMND JSYS that (very) loosely inspired much of the
subsequent command line notions in the industry, but this is far
simpler (the %COMND man page is longer than this code) and not in the
kernel... Also, it implements today's de-facto
command [verb]+ [opts]* [args]*
format rather than the old, archaic TOPS-20 command format :)

This is a snapshot of a work in progress to get the nvme passthru
stuff committed. In time it will become a private library and used
by some other programs in the tree that conform to the above pattern.

Differential Revision: https://reviews.freebsd.org/D19296