libsa: Make the nvlist implementation more self-contained

libsa: Make the nvlist implementation more self-contained

Move declarations into a new nvlist.h rather than putting everything in
libzfs.h.  This makes this nvlist code easier to reuse elsewhere.  In
particular, the nvlist implementation in sys/contrib/libnv does not
provide XDR encoding, but this is needed when reading from or writing to
ZFS pools.

Also:
- Remove references to boolean_t.  It has to be a 32-bit int here, so
  just reference the underlying type.
- Add includes needed when compiling the nvlist code outside of stand/.

No functional change intended.

Reviewed by:	tsoome
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D35255