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