stand: Separate base and cli parts of nvstore

zfs lives in libsa. However, it depends on nvstore (and other things)
that are in common. Fix part of this layering violation by splitting
nvstore into a libsa piece (which is the base implementation) and
keeping a much smaller common piece (to implement the nvstore
command). This just leaves zfs' knowledge of device names that's
specific to common and its calling platform specific init code to
resolve. Add a nvstore.h file for these two parts to communicate private
things and move the public nvstore api from bootstrap.h to stand.h.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D38043