Move nvmecontrol to using linker sets for commands

More commands will be added to nvmecontrol. Also, there will be a few
more vendor commands (some of which may need to remain private to
companies writing them). The first step on that journey is to move to
using linker sets to dispatch commands. The next step will be using
dlopen to bring in the .so's that have the command that might need
to remain private for seamless integration.

Similar changes to this will be needed for vendor specific log pages.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D18403