libzfs is the only user of libshare, and only internally, so there's no
particular reason to build it separately, nor to export its symbols. So,
pull it into libzfs proper, remove its "public" header, and hide its
symbols.
The bare minimum "public" API is just to count and enumerate the
supported share types. These are moved to libzfs.h with the other share
API.
Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes#18072
